I've been on those sites before, never really used them though, mainly just signed up out of curiosity or boredom.

I did have a random hook up on Adult Friend Finder though. I didn't put any effort into my profile, mostly just put a couple pictures of myself on there. Randomly, a woman in her early 30's (I was in my early 20's) messaged me asking me "why aren't we having sex yet?" Twenty minutes later, I was in my car driving to meet her at a hotel room. Got there, and she was already there in her car waiting. Got out, gave her a hug, she went and paid for a room, and then called her husband to let her know she was there safe. After some small talk, she changed into lingerie, and we had sex. She took a few pictures for her husband, none had my face in them. Afterwards, we hugged, kissed, and I never saw her again.

I don't consider Tinder to technically be a meet and fuck site, but I've met a couple girls off of it. One was on NYE and she happened to randomly run into her friend at the bar we were at, crying hysterically, so she ended up consoling her friend and I went and did my own thing. Another girl I met and things were going great, she was attractive, but I specifically had on my profile "nobody with kids" and she dropped that she had kids during the date. She invited me back to her place, and I declined and went back to my girlfriends house instead (girlfriend knew and was the one who suggested me going out with other girls).

I've had other girls that I had the potential to meet off of Tinder that I ended up deciding not to because they either didn't seem to grasp the concept of an open relationship, or seemed like they were going to be clingy, that they'd end up wanting to date exclusively eventually, etc.