My bet was going to be Atlassian. They have the clearest competitor to Campfire (Hipchat), a history of pulling off acquisitions (Bitbucket) and from 37Signals perspective could probably make a better case that they could take care / migrate their customers.
Not sure why anybody would want to acquire Campfire? It's an utterly terrible product. If GitHub wanted to added chat collaboration features they'd write them from scratch based upon WebSockets / Socket.IO... not the legacy polling that Campfire does!
GitHub are known to have invested a lot in using Campfire, for example their bot Hubot integrates really well and they use it to manage the majority of their infrastructure.
It's definitely possible but I wonder what Githubs strategy long term is and how it would fit into that. While I'm sure there are lot of people that use Campfire at GH (or maybe not), it may not be right for them to take care of and expand.
But Github is already doing what 37signals is aiming for and that is being a company focused on a single product. I'd think they want to maintain that.
They actually sold Gauges to Fastest Forward a couple months ago. Hopefully the new owners will give it some much-needed love; it's pretty, but... let's say unnecessarily minimalist. (If there's a way to get it to give me all the referers in a time period other than day-by-day, for instance, I still haven't figured it out.)
