I hate to say it...
But maybe this just shows the Americans are getting ahead of so much of the rest of the (idiot) world and recognising that having 400 'friends' on your list actually means squat, and the people you should really be bothered with are the ones with whom you genuinely have some sort of relationship.
At first, social networking can be exciting, and hundreds of people get added to your friend list. However, sooner or later people tend to start realising that they aren't all that tight with so many people and just can't be bothered to keep up with them AND find enough time in the day to perform even the basic chores, let alone go to work, etc.
So, sooner or later they either start cutting people off (because seriously, how many f---ing requests can you stand to send someone a f---ing pig on some farm game anyway?) and recognising that alot of them are just pumping cr*p out in a desperate attempt to get attention.
Eventually, many many people will really pare their lists down to the people the act they actually know and have any sort of relationship with, and advertisers will push to find even more ways to drop intrusive adverts on users' PCs.