My Little Pony: Friendship Is Magic fandom is  an animated television series produced by Hasbro
I've heard of it, I think the popularity is tied between the brand and quality writing and show design.
The series was tied in with the 2010 launch of dolls and play sets, and original programming for U.S. cable channel Discovery Family 
Seems like strategic planning on their part, Lauren Faust was also selected as the creative developer.
Under Hasbro's guidance, Faust developed the show to appeal to the target demographic of young girls and their parents
I agree, and think that she has made characters that challenge norms of typically "girly" images.
It also found an unlikely audience in a large group of adult Internet users in late 2010 and early 2011
Yes, I have heard that these fans are called brony or bronies.
yes and ha's because mostly male, were drawn to the show's characters
I find it so interesting that despite it's roots as a toy tie-in, the adult interest in the show is compared to sponge bob and others.