Windows is a bad analogy. There's no data involved. Facebook's data is open. It isn't federated like XMPP, which seems to be what you're requesting. Federation

Windows is a bad analogy. There's no data involved. Facebook's data is open. It isn't federated like XMPP, which seems to be what you're requesting. Federation would be a handicap for a product that changes as much and as quickly as Facebook has over the years. Instant messaging is simple in comparison.