The "fm" in the name Ask.fm is actually just a top-level domain name like .com or .net
When the guys who set up Ask.fm were looking for a domain name for their Q&A site, Ask.com or Ask.net were already taken.
But because the guys liked the idea of "fm" because it reminded them of fm radios which they listened to when growing up, they decided to go for the name Ask.fm
Now it is one of the most popular Q&A sites among teenagers, although there have also been problems to do with cyber-bullying on Ask.fm, where some people even committed suicide because it made them so depressed.