Maybe I'm naive, but why would turning off the gathering of information take a while? This reminds me of unsubscribing to email newsletters, where the final goodbye says something like "you should stop receiving our emails within 6-8 weeks."
Thats a awfully cautions attitude and smells like a huge cop out for the well known fly-by-the-seat-of-your-pants commit to live strategy that facebook has.
I'm only defending them because it annoys me when people who aren't familiar with the software internals tell me "this is a minor change, it should take you less than an hour".
Essentially, instead of FB like widget -> request to facebook I would think FB like widget -> add to local datastore.
Then FB could do an optimised/aggragated query on the local database. The only thing would be that it would introduce large latency in the resulting data if its sent back only on FB login.