There is an argument to be made that if the company has more than a % of marketshare in a given market or feature and intentionally prevents interoperability then it indeed owes services to everyone.

In this case, Facebook is the new website/storefront/phone number for a lot of small businesses as well as the default communications medium for a large chunk of the population and makes it impossible to interact with these without having an account (unlike let's say a telco or email provider which will happily receive contacts from other providers).