Although I would consider it "nice to have" if the user could authenticate with Facebook or LinkedIn, for clients and staff we need an email in the account, so email providers would serve not only as authenticators but also to gather an active and trusted email address and link the account to that, so I would focus on Google, Gmail with > 600 M users and Google Apps with > 5 M companies.
If you are planning to add it for any OAuth service provider, I would suggest then to have settings to allow or disallow each one from a list of providers supported.
I would personally leave it only for Google and Hotmail, in that case, maybe Yahoo! when it gets to OAuth 2.0.