This is already happening with lots of small business websites -- they are only on Facebook, so you can't eg. look at the menu without a Facebook account.
Or company help desks, at this point after twitter changes its licensing lots of companies quit it, it was the only social media account I still used for chat-like help. Most now offer Facebook but under no circumstance am I going to create another account. Now I'm stuck with calling and waiting in line since they also stopped doing email (only callback service, which rarely works).
