Kasidie
Kasidie gives us the old Facebook, but for swingers type of vibe. It's got a TON of users on the platform and you can meet people from all over the world. We love the ability to see who's viewing your profile and where they're from, send them a flirt (similar to the Facebook "poke" feature) and to have private photo albums so you can regulate who you allow to see more of you. You can also view people's upcoming travel plans AND events going on near you or in a region you are planning a trip to, which we really appreciate because we love to travel so often. We were reluctant to pay for a membership at first, but definitely have no regrets after. Highly recommend.
SDC
In our experience, SDC seems to be far more popular with people in southern states and on the east coast of the US. The interface is quite different than Kasidie's, but there are a lot of the same features available, which we appreciate. Private photo albums, see who's seeing you, upcoming events, etc. Like Kasidie, It's also a paid membership site, but well worth it if you are fully invested in the LS.