Edit: From the net upvotes my post has (22) and the net upvotes that the top  disagreeing comment (29) has, I estimate about 51/(51+29)=64% to be on board with a movement-wide transition to Signal. Still, I have to admit, I underestimated the level of opposition to this proposal.

There are benefits to all EAs being on the same messaging app, so it's good to do an exhaustive cost-benefit analysis before any equilibrium shift.

I think the pros of Signal are

1. Privacy/security against data breaches, which threaten reputational harm to the EA movement.
2. You can add 1000 people per groupchat (Facebook Messenger can only add 150 people).
3. Minor reason: The profits fueling Facebook AI's capabilities research could be reduced if many users of their core services leave. But it is unlikely that the EA community will be the tipping point of this trend.

and the pros of Facebook Messenger are

1. You can search for the name of a person to add to a groupchat.
2. Your message history is on Facebook's server, and thus can be easily accessed across devices.
3. Minor reason: Facebook Messenger Rooms allow up to 50 people in a call (Signal allows only eight). But other video chat apps, ideally with high-privacy, can be used instead.

Expanding a point in my post: For the Signal equiilbrium shift to be effective, it is not enough to use Signal oneself. One should also take proactive actions to ensure that the EA community as a whole uses Signal for internal messages. For example, one could 

1. proactively make Signal groupchats for their EA circles/social events,
2. suggest switching to Signal when another EA proposes a Facebook-Messenger groupchat, and
3. create Signal groupchats for the subsets of privacy-prioritizing EAs, even in EA circles that have traditionally used Facebook Messenger.

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

Acknowledgements: I am extremely grateful to Michael Einhorn for this idea, and to Samuel Chen, Stephen Fowler, and Luise Wöhlke for their feedback on this post.

tl;dr. Facebook Messenger is dangerously insecure. It only takes one data breach of one insufficiently careful EA to harm the movement. Thus, all EAs should use Signal instead of Facebook Messenger for their internal messages.

The Post: In my experience, groupchats among effective altruists usually happen on either Slack (for work-related settings) or Facebook Messenger (for social settings in general). I've heard that the EA communities of Germany and the U.K. also use Signal, which I propose is a more impactful choice.

As a rule of thumb, all Facebook Messenger conversations between effective altruists should be moved to Signal.

How it works: "Signal is designed to never collect or store any sensitive information. Signal messages and calls cannot be accessed by us or other third parties because they are always end-to-end encrypted, private, and secure." (quoted from their website)

By default, Facebook Messenger is not end-to-end encrypted. Messages sent on the default version of Messenger are stored on Facebook's servers, which can be accessed by Facebook, law enforcement, and hackers. And Facebook has confirmed that it spies on Messenger conversations.

There is a (highly unknown) encrypted version of Facebook Messenger called Secret Conversations. There is also WhatsApp, a Facebook-owned message app which claims to have end-to-end encryption. However, even messages on WhatsApp can be accessed by Facebook and by law enforcement. This is compelling evidence that Facebook cannot be trusted with our messages.

Moreover, using any of these Facebook-owned message apps implicitly endorses, and potentially results in training data for, Facebook's current trajectory. This is a trajectory that can plausibly result in a castastrophe (even human extinction) due to an unaligned artificial intelligence developed by Facebook AI in the future. 

In contrast, Signal is a nonprofit whose mission is to provide open-source privacy technology to the public. We know that Signal does not store any data beyond a name and phone number (optional), the time of account creation, and the last log-in time. This has been verified by legally binding subpoenas.

Question: "What if I have nothing to hide?" This belief would most likely be overconfident. If effective altruists' messages are hacked, taken out of context, and publicly revealed, it could substantially and even permanently harm the movement. Consider the example of John Podesta, chair of Hillary Clinton's 2016 presidential campaign. Many of his emails, including those that made Clinton and her campaign look bad, were obtained by hackers in a data breach and published in Wikileaks. This event is likely to have at least partially contributed to Clinton's loss to Donald Trump. 

Question: "What if I'm very careful about what I say on Facebook Messenger?" Even if you are very careful, other effective altruists may not be. It only takes one data breach of one insufficiently careful EA to harm the movement. So, effective altruists who perpetuate our community's use of Facebook Messenger are forcing a painful choice between risking reputational harms (e.g., from a data breach) and sacrificing epistemics (e.g., if the whole EA community walks on eggshells in order to not look bad in the event of a data breach). We can avoid this painful choice by agreeing as a community to use Signal.

Question: "Isn't Facebook's event function very useful to make EA events known to the public?" I completely agree, and think it may often be a worthwhile tradeoff to use Facebook's event function for EA outreach (especially towards the comparatively older people, those outside of Generation Z), even if this would yield a marginal amount of training data for Facebook. But we should avoid using Facebook when we don't absolutely have to. This means we should use Signal for all internal messages between effective altruists.

Importantly, switching to Signal has no comparable costs. The only cost I can think of is that the UX (User Experience) might be slightly better for Facebook Messenger than Signal. Even this is subjective. Some would prefer Signal's UX: for example, the ability to easily and safely access all effective altruists in a groupchat and their phone numbers after being added to it. (Note that this only applies to people who prefer to share their phone numbers. People who don't want to tie their phone number to their Signal accounts have many ways to avoid doing so.)

One ostensible advantage of Facebook Messenger is that connecting with EA acquaintances is (in some situations) marginally easier, because you can search for their names. But this effect is truly marginal; we are talking about a difference of at most several seconds per connection. And if these several seconds per connection are invested in adding yourself to an EA groupchat, Signal guarantees that you have an easy and safe access to all effective altruists in the groupchat and their phone numbers.  This would aid the EA community's social connectivity in the long run, making even the UX a point in favor of Signal.

To quickly add someone to a Signal groupchat, all you have to do is create an invite link and send it. This method, and the alternative method of using a QR code, are detailed here.

Is a community-wide transition feasible? In the past three days, we have achieved an almost-perfect success rate for persuading effective altruists to transition from Facebook Messenger to Signal. At our EA house/coworking space in Berkeley (informally named Prospective), we have successfully transitioned from our previous groupchat in Facebook Messenger to a new groupchat in Signal, with over 90% supporting the change. In fact, I frequently heard comments like, "I didn't use Facebook Messenger until I came to Berkeley to work on EA stuff, so I was forced to download it."

It is, of course, a nontrivial problem to change the entire EA community's equilibrium away from Facebook Messenger and towards Signal. Even though Signal constitutes a better long-term equilibrium for our community, there is already a critical mass of effective altruists who use Facebook Messenger for messaging other effective altruists.

To effectively undo this network effect and reestablish it around Signal:

1. We should explicitly encourage every fellow effective altruist we meet (at university groups, conferences, and social events) to, as a matter of principle, use Signal rather than Facebook Messenger.
2. We should crowdsource a shared document (here) detailing how to transition to and optimally use Signal. Please let me know if you would like to collaborate on this!