How they compare
Both can lead to real-world meetups, but they work very differently.
FetLife has been where the kink and BDSM community gathers online since 2008: profiles, discussion groups for every fetish, and listings of local munches, parties and events. It's free, paid for by optional supporter memberships.
AdultFriendFinder is the longest-running hookup site, with profiles, messaging, chat rooms and live cams. Browsing is free, but messaging needs a membership: about $40 for one month or about $20 a month yearly.