I wonder if Yammer could issue some sort of legal covenant not to sue that would be binding even if they sold the company or the patent changed hands some other way? It looks like Yammer's CEO filed a patent as recently as last August:
A system and method for collaborative short messaging and discussion are described. According to one embodiment, a computer-implemented method for collaborative short messaging and discussion, comprises grouping users into client networks based on existing shared attributes. System resources are partitioned for messaging across client networks. Users in a client network are allowed to view or respond only to messages within the client network.
I know it's just a summary, but this sounds pretty much exactly like the early days of Facebook when user's were confined to their college "network".
IANAL but people who Are A Lawyer seem to feel that there's no legally meaningful way to place anything in the public domain, currently, at least in the United States.
Write up what you did and publish it somewhere. It's not magic - patent examiners would still need to find what you did, and if the patent is eventually granted and someone sued, it's always expensive to deal with lawsuits even if you're in the right. But if you clearly state what you did and put that write-up in some place that is accessible by others and clearly dated, then you've established that others should not be able to patent it.
The public domain is the wrong place for this, but you can certainly grant irrevocable patent licenses. IIRC this is how the GPL3 works. (Not that you'd have to license under the GPL to use the same strategy, it just shows it's possible)
http://www.google.com/patents?id=D-r_AQAAEBAJ&printsec=f...