Six Degrees of Cooperation has a well-balanced article on enterprise social networking.  First page is chock full of a little too much vendor rah-rah, but pp. 2-3 have some good details, esp. around some of the organizational challenges.

A couple of good quotes:

"[Enterprise social networking systems] make the most sense in industries in which deals are big and
relationships are a key differentiator. Companies that are spread out
geographically with a large number of contacts are especially good

"There are trickier considerations, though. Privacy is the biggest.
People don’t want Big Brother reading their E-mails, handing out
ex-girlfriends’ phone numbers, or revealing confidential client
information…[one firm] worries about a revolt if it mines company address books, but has heard few complaints about E-mail scanning."

(Hat tip:  Scott Allen)