I've said this repeatedly: Facebook is a high-risk proposition, particularly as an investment (disclaimer: I work for Google). "Intent" as other commenters have noted is a big part of the story but it's not the only story. Search can broadly be broken up into at least two categories: 1. Navigational eg typing "b and h" into the browser; and 2. Informational eg "cheap flights to jamaica". I don't know what the make up…
I would argue that things like the massive photo collections people have made provide a large level of lock-in. I also suspect that Timeline is the first play in a "all your history, here" move that would make moving to another service very difficult.