I don't see gamers having less friends than non-gamers at all. In fact, it might cause them to make some more due to having a sort of connection or having common ground with other people also involved with video games.
Yes, I also find the idea that gamers have less friends. Especially since in many cases gaming actually brings a large group of people together, both online and in real life. Where I've gamed with friends of mine many times growing up, even having a LAN party with my local Star Trek Club. Diabo II, Minecraft, WoW, and Quake (Though I wasn't as good playing the last game).
Well, in my experience, not really, in fact I think that gamers have more friends than non-gamers, just because of the fact that gaming is becoming more relevant that it's rare to find out that one guy or girl is actually not playing games.