They spend years here and just leave one day?
but it's (mostly) not like that
you can't see it because you're not on FB
what I see is people gradually spending more and more time there and less and less time here
littlek, dasha, reyn and shewolfnm are examples that spring to mind for that pattern
sometimes I think they don't realize they're not at A2k at all anymore because we still see each other's postings and photos there
other people return here and pretty much abandon their FB accounts over time - Frank Apisa and verbivore fall into that camp