Top social networking sites like Facebook and LinkedIn may be well-established, but that doesn’t mean they’ve stopped growing.
Nearly half of the biggest social networking sites are also among the fastest growing, according to data released Wednesday by Nielsen Online.
These top sites have also expanded beyond the youthful audience the social networking space initially captured — to find a place in older visitors’ professional lives.  Facebook, LinkedIn, and Reunion.com are most popular among visitors ages 25-34, 35-49, and 55-64, respectively, Nielsen reported.

Rank
(by Sept. YOY UA growth)
10 Fastest Growing
Social Networking Sites: Sept. 2008
Sept. 2007:
Unique Audience …