EDIT: To explain and not be snarky, QQ is an IM/portal/social network/web mail/social game platform that is actually larger than Facebook. (Over 600 million active last I checked.) They completely dominate every aspect of Chinese web software.
There would be many more using Facebook if it wasn't blocked. I'm not sure how it's doing now but I remember lots of students last year were joining xiaonei which is just a facebook duplicate for Chinese students only.
IMO many of these sites are blocked in China so Chinese companies can dominate the domestic market rather than for political reasons.
EDIT: To explain and not be snarky, QQ is an IM/portal/social network/web mail/social game platform that is actually larger than Facebook. (Over 600 million active last I checked.) They completely dominate every aspect of Chinese web software.