But smaller screen sizes than PCs, less sophisticated ad targeting and wariness by advertisers is holding down ad rates or spending on mobile devices.

WSJ: Twitter's Mobile Ads Begin to Click

Using that knowledge, Fab is targeting its TV ad buys to find the same demographics that it has been targeting through Facebook.

FORBES: Fab.com Runs $8M TV Ad Campaign, Expects $25M In 2012 Facebook Ads

How much the ad targeting has helped Facebook's business is unclear, since the Menlo Park, Calif.

WSJ: Buy Signal: Facebook Widens Data Targeting