AOL announced today that it will pay $850 million in cash to acquire Bebo, a social networking site.

Bebo is a 100-employee operation based in the U.S. and Great Britain, with the majority of its 40 million-membership concentrated in the latter. Its users constitute about 3% of the combined social-networking audience of MySpace, Facebook and Bebo, according to metrics from comScore.

Facing steady declines in subscriptions and sluggish advertising growth, AOL recently began an overseas membership drive with plans to expand to 30 countries outside the U.S. by the end of this year. AOL said it will feature Bebo prominently in this launch following the close of the deal.

AOL President Ron Grant explained that Bebo’s large international contingent will help AOL expand internationally, and that AOL will help Bebo reach the United States. “This is going to be the cornerstone of our strategy to really go after the social media space,” he said in a statement.

According to comScore, Bebo had 22.4 million unique visitors worldwide in January, with 60% of its traffic from Europe, 22% from North America and 16% from Asia-Pacific.