Of course it is – you've got this back to front. The grant of additional rights to use any patent Facebook may hold is an additional benefit and in no way places any restriction on you.
If you're going to worry about anything, worry about the squillions of open-source projects that don't include any patent grants. But you probably shouldn't worry about that either.
Most open-source projects' owners and contributors don't have any patents to enforce, thus can't grant anything. The rest tend to be less chilling than the clause that Facebook uses - there's no "we'll only grant you this if you promise never to mention that any of our patents might be invalid".
Are you certain that Facebook doesn't have any patents that cover any of its open-source software?
If you're going to worry about anything, worry about the squillions of open-source projects that don't include any patent grants. But you probably shouldn't worry about that either.