[superuser2]

>claimed that one of their patents was invalid

You would (probably) only do this defensively against an aggression by Facebook. People don't generally have an interest in alleging patents are invalid, unless they are being sued for infringement of those patents.

[dynamic]

Some people do! Check out the top story on HN right now, which is about EFF invalidating a patent held by a patent troll: https://news.ycombinator.com/item?id=9356767. Nobody was suing EFF (admittedly, that's almost a technicality since the patent holder was suing others, but EFF was trying to advance the public interest).

Also check out Ask Patents, which works on invalidating patent applications: http://www.joelonsoftware.com/items/2013/07/22.html.

[joesb]

Still, it means that, if Facebook sue you with stupidly-obvious-invalid patent on entirely unrelated product, say a patent that car must have a glass windows, then you can't even defend that such patent is invalid.