A person is communitied-out when they have reached the limit as to the number of online communities they can participate in.
Every forum, message board, wiki, company shill site, article-voting/linking, and photo-posting web location is now a "community", to some extent. Participation as a community member can be fun and rewarding, but there are just so many of them. Feeling communitied-out is something that comes and goes. Sometimes the person just needs a break from a given community for a while. At other times, though, it is pretty serious. Some community is going to have to fall off the Web before the person can slot in a new one.