Sometimes I think buyers like to wait a few days or maybe even a week to be sure that the item is all that it was advertised to be before leaving FB.
When I was selling alot, I would sometimes wait until the end of the week and do maybe 20-30 at a time.
On a final note, I once had a seller (newish) email me the day after I received my purchase from her and in no uncertain terms DEMANDED that I leave her immediate FB as she was trying to establish herself. I've only run into that situation once, thankfully, but thought it was totally rude and inappropriate!