For my latest product, Grove, I wanted to allow users to quickly log in or sign up with an existing account, such as Twitter or Facebook.
A while back I collected some data to determine how many people were using Facebook, Twitter, and OpenID to log in to TypePad. It was fairly obvious to me that people liked logging in using existing accounts.
So our team created both GitHub and Twitter log in buttons.
With TypePad, the most popular service for logging in was Facebook. However, I purposely didn't add a Facebook log in button because I felt that our target audience (developers) wouldn't be as likely to use it. In fact, they might actually find a Facebook button a turn-off.
Our team isn't collecting much data yet, but we do know how many accounts total are connected via GitHub or Twitter.
Users who have connected their account:
Twitter: 13.96 %
GitHub: 20.41 %
Wow. This means that 1 in 5 users have probably logged in with GitHub while only 1 in 7 have logged in with Twitter.
I guess I don't need to add that Facebook button any time soon.