How they compare
Both are for the kink and BDSM community, but they work very differently.
FetLife has been the community's online home since 2008: profiles, groups for every fetish, writing, photos and listings of local munches, parties and events. It's free, paid for by optional supporter memberships, and it isn't a hookup app, so finding play partners takes effort.
ALT is a fetish dating site from the FriendFinder family, with around 2 million members claimed and search across 70 fetishes worldwide. Joining is free, but messaging needs a paid upgrade, and many user reviews report fake or inactive profiles.