There are obvious advantages to limiting sign-ups to a service like Facebook connect - getting a name, age, avatar, etc. without the annoyance to your customers of asking them to fill it all in.
We are currently considering this, but there are some good arguments for using other services as well - such as Twitter, LinkedIn, Google, etc. I can even think of some very good reasons (privacy being chief amongst them) to still keep our own sign-in.
The issue then is that giving people too many options is not a good UX.
Our target audience is subject specific (architecture, language learning, etc.) so we can't predict our customer audience very well yet.
What would you use for sign-ups and why?
Edit: I need to add that our application is targeted at mobile use, so screen real estate is also a consideration in showing many options.