I see world wide Bahai rank somewhere between 9th and 13th in volume of participants...depending on the report.
As to the Wiccan increases...and 3rd largest by 2012...I'd like to say I've got some swamp land in Florida..but truth is I wish I would have purchased some back then...it is worth a fortune...and they did sell someone London Bridge...so never say never...
Despite that, we are talking 6 short years away and my money will ride on the 'it'll never happen boat. Now I'm not saying never happen, just not 3rd by 2006. Yes, they are rising strong and will continue to do so as there are many souls that don't go conventional but are looking for something to hang their hat on. But there are only so many of those, and the doubling every 18 months won't continue...there does exist a curve to these sort of things. Also there are already internal rifts and issues which will soon lead to splinter groups and attrition rates having to be included over the next 6 years as well....
I think currently Wicca is one of the quite likely places to end up for many of those that revolt against their upbringing.