I've been on Guru, Freelancer, PPH, Fiverr, Twine and some dedicated to translators specifically. I use them for daily memory training: "Now, this job posting sounds identical to the one I read on the other platform ..."
What I would really like to see is a study to determine which platforms clients, large and small, actually know or recognise. Because there's no point in sitting around somewhere if people can't find the place, is there?