Is there any limit or fee to use the Facebook API? Can I develop a commercial software app with the Facebook API?
This is best answered already on StackOverflow:
According to Facebook dev policies "I. Features and Functionality" the only limits are:
(>5M MAU) or (>100M API calls per day) or (>50M impressions per day).
(Mau means monthly users)
The best answer to this question from another forum
"After some testing and discussion with the Facebook platform team, there is no official limit I'm aware of or can find in the documentation. However, I've found 600 calls per 600 seconds, per token & per IP to be about where they stop you. I've also seen some application based rate limiting but don't have any numbers
You can develop commercial software with the API, read the dev policy for more details. If you exceed the limits above you'll probably have to pay.