> If you do want to open source it, I highly recommend GPLv2 or 3.

This is solid advice for client software. However for software to run on the server side, AGPL providers better protections to ensure further developments remain free (copyleft).

While the AGPL would ensure that developments remain free, it will also ensure that most entities (commercial and non-commercial) that need tight integration would not touch it with a 10 foot poll.

The choice of license really depends on what the OP aims to achieve with his software.

> it will also ensure that most entities

That is simply not true. It may be true for huge corporations with legal counsel spreading FUD. But why should we care about these people ruining our lives? They're already destroying the planet and making sure millions of people are enslaved in the mines/factories across the third world, and we should provide free work for them?!

Interesting entities (libraries, schools, associations, workers coops) will not care about the software license.

Unfortunately, it is. The AGPL exposes licensees to the extraordinary risk of having to disclose any private internal/infra code that integrates with it and, more importantly, the legal and technical costs of doing so.

I assure you, even "interesting" entities care about this. Hell, even public institutions I worked for did.

In this case, wouldn't it be great if the big dogs picked up the tech and ran with it?

The same way Google and Facebook picked up XMPP and ran with it?

Remember that they failed though.

Looks dead enough to me in practice?

From my perspective that was a good thing. I still use Pidgin as my google hangouts client.

It was a good thing until they ruined it after increasing their strangle on the userbase. It would be sad to see the existing big guys get involved in Matrix to make it popular, only to corrupt it and close it off once it's served its purpose.