Newsvine, which currently has 1m unique users a month, said it welcomed the marriage with MSNBC, which has 29m unique users, because it would spread the idea of participatory news as far as possible.
The start-up site allows users to post published news stories from any source, which can be discussed and ranked according to their popularity with other Newsvine users, and also to write their own articles.
Newsvine will continue to operate independently, while MSNBC will study the possibility of adopting Newsvine's features and user-generated content.
Newsvine was founded by former Disney and ESPN executives with $1.5m in 2005 and backed by venture capital money. Its six full-time employees include chief executive Mike Davidson, who said the deal would take the social media site to a wider audience.
Davidson said: "We founded the company with the notion that big and little media can interact in a way that respects established journalism and empowers the individual at the same time.
"We're thrilled that even more people will now be able to contribute to the site and help further the cause of participatory news and citizen journalism."
Similar Web 2.0 news start-ups have also been bought up over the last 18 months, with Del.icio.us bought by Yahoo!, Reddit by Wired.com, and Stumbleupon.com by eBay.
Last week, in another link between the new and old, the US political blog and news site Huffington Post recruited Betsy Morgan, CBSNews.com general manager, as its chief executive on October 2.