I was in Fictionpress first since 2010 but I changed pseudonyms moved to Wattpad and then remade a new account in Fictionpress but didn't follow anymore. Fictionpress seems stricter in posting stories like you have to wait for the posts to be up and it was really harder to get reviews there. The more chapters you add to your story without the orange "reviews" beside it, the lesser are your chances to get anymore in the future. Most people there just check the stories already with some reviews on them. So it's really all luck.
What I only liked abut Fictionpress is that there were more mature writers and I've read really great engaging pieces there. They all probably moved somewhere else.
I agree, Fictionpress did have some mature writers. I follow someone who has written over 300 stories and completed them (well, 95% of them) and they are pretty good. Inkitt has a lot of mature writers who take writing seriously so I think lots of people went there including myself :)
What I only liked abut Fictionpress is that there were more mature writers and I've read really great engaging pieces there. They all probably moved somewhere else.