There is absolutely a market for F/F. I have yet to attend a Pride event without hearing, at least a hundred times, "Why aren't there more lesbian books?"
Bold Strokes, Bella Books, and Riptide are actively looking for F/F, and there are readers who want it. I said above that my F/F books have sold comparably to my M/M -- to be more specific, they've both sold in the neighborhood of 5,000 copies (they came out in December 2014 and 2015). And that's under my definitely-not-as-well-known pseudonym (hetero and bisexual books under that same name don't sell for crap).
So...yes, y'all. Write F/F and publish it. There are readers, and they are hungry!