In a statement Monday, Facebook said it was surprised by the move.

"We're disappointed that Yahoo, a longtime business partner of Facebook and a company that has substantially benefited from its association with Facebook, has decided to resort to litigation. Once again, we learned of Yahoo's decision simultaneously with the media. We will defend ourselves vigorously against these puzzling actions," it said.

It's likely no coincidence that Yahoo filed the suit now, just before Facebook is set to go public. "I think the timing coincides with the IPO and they're going to try to use the IPO as a source of pressure to get them to settle rather than litigate," said Greg Sterling, an analyst with Sterling Market Intelligence.

The strategy echoes Yahoo's decision to sue Google just before that company filed for its IPO. Yahoo sued Google for allegedly infringing patents controlled by Yahoo subsidiary Overture. Onlookers thought that dispute so that it wasn't involved in any potentially costly legal action when it went public.

In the suit against Facebook, Yahoo argues that simple back payment of royalties won't suffice.