AOL, which bought Bebo in March 2008 for $850m, has its own advertising business, Platform A, and is now reviewing its options for Bebo's future ad sales arrangements. Its deal with Yahoo! expires this year.
Yahoo! has handled advertising for Bebo since 2007. The deal was Yahoo!'s first with a social networking site, as it aimed to target brands seeking to reach 13- to 24-year-olds.
Mark Rabe, managing director of Yahoo! UK Sales, said: 'We've built a successful partnership with Bebo.'
An end to the Bebo deal would come as a further blow to Yahoo! following Vodafone's decision to end its exclusive mobile ad partnership with the company.
Bebo's in-house sales team focuses on custom sponsorship campaigns and branded original productions. It is currently finalising its next ad-funded content production with O2.
Separately, Bebo is trialling mobile advertising with brands including Coke. The Coke ads enable users to send a virtual bottle of the drink, which generates them Bebo 'luv'.