Here's one I just set up in < 2 minutes:
The problem is that we're not just using Twitter for sign-in and as a source of friends: we're also using it as a conference speaker identity directory. Lanyrd lets you list the speakers for a conference, e.g. "@timbray is speaking at AndroidCon" - without those people necessarily having signed up for accounts on the site. Since speakers often have large Twitter followings, this makes it much more likely that a user signing in for the first time will get at least a few conference suggestions.
This complicates things a bit when we start involving other identity providers, since we then have to start tracking equivalent accounts on multiple services (Tim Bray on LinkedIn = TimBray on Facebook = @timbray on Twitter). Not impossible, but fiddly enough that we're sticking with Twitter for the moment.