When I see that, I figure you're making an association with Facebook (or twitter, or whatever) on purpose. You're targeting the same audience as those who use that other service, and as such will likely add or alter features based on what that target audience wants.

I don't use Facebook much, nor twitter all that much either, and don't see myself as part of (real or imagined) those demographics. So I'm skeptical that your service is for me, or will follow my interests.

I also wonder if your service will be posting updates or tweets on my behalf ("I just joined a service that tweets things without my wanting them to!") or if at some point they'll be an unwelcome exchange of info among applications.

While I'm sure some people see the option to use an existing ID system as a real convenience, I see it as a potential pathway to privacy invasion.