How they compare
AdultFriendFinder is the longest-running and best-known hookup site, and closer to an adult social network: beyond profiles and messaging there are chat rooms, member content and live cams. A free account lets you browse; messaging needs a paid membership, around $40 for one month or about $20 a month on yearly plans at the time of writing. Expect upsells.
Ashley Madison is built for discreet affairs, with privacy features to match. It's free for women; men buy credits to start conversations, with packs from about $49 for 100 credits up to around $249 for 1,000.