r/BDSMcommunity is a Subreddit for BDSM with roughly 452K members. It uses a forum format for communication.
In their own words
The BDSMcommunity subreddit is dedicated to discussions, questions, and sharing experiences related to BDSM and kink lifestyles. You must be 18+ to view this community
The CageFriendly, Fun, Kink Positive, BDSM Community. The Cage is a community and social networking platform focused on BDSM and kink-related discussions.
Fetish.comKink-positive BDSM community for fetish dating with free chat for kinksters. With 24/7 forums, chat and magazine, you'll always be entertained!
FetLifeFetLife is the most popular social network for the BDSM, fetish & kinky community. Its mission is to help people feel comfortable with who they are sexually. It's important to note that while FetLife is a popular platform for people in the BDSM and kink community, it's intended for adults only and requires users to be 18 years or older to join.
The Bound ForumA really informal and non-threatening place for kinksters with a taste for all areas of the BDSM lifestyle. The site is designed to be less ‘in your face’ than some bondage sites and is heavily text orientated. The Bound Forum covers a wide range of kinks and has a story section for budding authors or readers to really embrace their fantasies.
BDSM-ZoneBDSM-Zone is another free to register BDSM community website. This site is devoted to everything which is related in one or another way with BDSM.
BDSM.comBDSM offers a rich and varied community experience with over 32,000 groups to join offering more than 2.1 million discussions. There are bondage groups arranged by special interest and kink as well as by location meaning that you should be able to find one that suits you.
Kink TalkKink Talk is a community and resource for adults researching and exploring the kink, BDSM, and fetish worlds. Here, you can discuss BDSM, Femdom, Chastity Belts, Bondage, and more!