Three things strike me about this map:

1. There are networks (Myspace, Bebo & Friendster) which seem long gone, but are still bringing in the people
2. Habbo is often forgotten, but is the 2nd largest network in the world
3. There are a growing number of specific utilty / thematic sites

Massive hat tip to the Digital Buzz Blog

Massive disclaimer; the company I work for, works for Habbo.