But using those services to check into the applications running your business? Fuck no. I'm certainly not going to let anyone depend on their ability to get paying work done by whether Twitter is up or not. And I know of plenty of people who aren't interested in mixing their private-life Facebook with their work-life accounts.
Then of course there's Google. I'd be weary to let a large number of customers be owned by that Gorilla.
OpenID was promising because it was an open standard, not controlled by any one party. But unfortunately it had the usability of your average open source project (acceptable for hackers, terrible for anyone else).
This might be the good middle ground between facebook login and an entirely new login... a facebook-assisted signup procedure.