Give users something to do!
December does slow down as access to broadband decreases when this audience goes home for break. They also spend time w/family and friends 'offline' (wateva!).
I do believe that people have gotten bored of social networks, which accounts for FB and now MySpace and others opening their networks to 3rd party applications (i.e. OpenSocial) to keep people on their sites (remember the old buzzword "sticky"?).
Social Networks will either go niche or revolve around something to do instead of hope that people find interest in hyperlinking between people they don't know in search of good content and people.