What is the fastest growing religion in the USA?

I recently received some spam email claiming that Islam is the fastest growing religion in America. I find this to be a suspicious claim, but I’m not really sure. So, there are different ways to define this:

* The religion whose absolute number of adherents is growing the fastest (by whatever means).
* The religion which is growing fastest in terms of percentage growth per year (by whatever means).
* The religion which is gaining the greatest number of converts.

If anybody has any actual data from a respectable source, please let me know. I am interested in any of the above definitions of fastest growing.

Thanks!