> FB would be able to counter-sue for copyright infringement due to the suing company's use of FB's open source
the whole point of mit/bsd/etc is copyright grant. fb can only realistically win a suit wrt infringement if they revoke the mit/bsd license first (not sure if this is possible...not sure if it's even been tried before).
But since there is no patent grant with the MIT license, using the open source software is–potentially–open to patent litigation. I don’t think this has been tested in court yet.
In that case, there are two issues that need to be covered by FB; first, they need receive an agreement from all collaborators they agree with the license change—most likely, the SLA takes care of that; second, forks of the code up until the license change point still hold the original license—any fork that was created before the license change remains with the original license, and so does every use of code that consists only of code before the license change. This is what I meant by “retroactively”.
