I am a rare visitor to FS and since it is a newer site I wouldn't be surprised if habit keeps a lot of the older (in age and/or membership) readers here, even though FS has stories on there not available on SOL.
I wouldn't assume the readers are that much different, other than the kids too young to access an 'Adult' site. I suspect a lot of regular SOL readers visit FS because they don't have to wade through the stroke and "much sex" stories to find the more literary stories (that explains how so many of my readers read my stories there).
I typically, if I can, produce separate versions of my stories for SOL and FS, with any sex and much of the language removed, although I still rate the story for high school and above. However, I leave much of the four-letter words intact. FS cleans them up, but does so in such a way you can tell what was intended without displaying the word, which I like. You can tell when someone is upset, shocked or outraged.