You should know this by using mathematics. Do the following:

1. What age range is your primary consumer?

2. What country is your primary consumer?

3. What is the intersection between facebook users and the two questions above?

4. Based on whatever statistics you can find - how many users get turned away because of sign-up problems?

5. How many consumers will your initial marketing push reach - i.e, the consumers you hope to gain in the period when you don't have hard feature choices to make?

Intersect those 5 criteria and estimate what the benefit of facebook connect will be. You may want to contrast it with the benefits of the other features. Understand your numbers before spending time on stuff.