Slide executive Keith Rabois, one of the many new converts to Facebook in Silicon Valley, asks how much a Facebook app user is worth. The increasingly popular social network lets other Web companies plant applications, like Slide's photo-sharing service, on its site. But it's not clear how much loyalty users of such add-ons have to any site besides Facebook. So Rabois's question is apt, if a bit tardy: One wonders why Rabois didn't ponder such matters before Slide CEO Max Levchin got up on stage with Facebook CEO Mark Zuckerberg to swear fealty to Facebook's platform.