Open source involvement is definitely a PR move. When was the last time the open source community took something Google open-sourced, improved upon it, and then Google used those improvements? For the most part, these open-sourcing arrangements from big companies turn out to be one-sided, and so they're essentially buying good will in the hacker community. In Facebook's case, it also suggests that smart people get to work on cool stuff, so it helps for recruitment, too.
