Newsvine, a Seattle-based social news start-up has been acquired by MSNBC, a joint venture of Microsoft (MSFT) and GE (GE), for an undisclosed amount of cash. The deal closed on Friday and was supposed to be announced on Monday.

We officially became part of the msnbc.com family on Friday, October 5th but we’ve been talking since May.

Newsvine is going to be run as an independent company. Mike Davidson, one of the co-founders of the company says that the company could have gone out and raised a new round of financing and grown the operation, but instead decided that working with MSNBC was a better course of action.

Newsvine came out of stealth in November 2005. (Read, Introducing Newsvine.) I think we will expect more large media players will start buying these Web 2.0-Media start-ups as a way to bolster their web operations. News Corp. (NWS) was the first company to expand into social news when it acquired Newroo.

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