Social news site Newsvine, which launched in March 2006, has been acquired by MSNBC.com, a fifty-fifty joint venture between Microsoft and NBC. This is MSNBC.com’s first acquisition.

The deal, which closed on October 5, was all cash, but the acquisition price is not being disclosed. Both companies are based in Seattle. Mike Davidson, the founder of Newsvine, said that the companies will continue operating separately but that technology integration will occur over time. MSNBC has around 200 employees; Newsvine has just six.

Newsvine reaches this liquidity event after having raised just $1.5 million in capital. It joins both Stumbleupon and Mozy , each of which were also acquired earlier this year after having raised less than $2 million in venture capital.

This brings the total number of meaningful acquisitions of social news sites to three: Fox bought Newroo in March 2006 for $7 million (it is currently MySpace News) and Condé Nast, owner of Wired, acquired Reddit in October 2006.

Newsvine was recently mentioned on the Daily Show when Davidson “enhanced” Senator John McCain’s Myspace page.