The most recent one was Ashley Madison. But they suffered from a really huge privacy scandal, where hackers dug out information about their members and posted it for the public to read. Why would you want to engage in couples dating in the first place? It can get very complicated and messy especially when the emotions of more than 2 people are involved.
Try OkCupid.com. This website started about 10 years ago, and was developed with the angle to help people find their perfect match using scientific algorithms. While I cannot vouch for whether their science did work for me in getting the perfect partner, I did get to know a lot of friends from that website and even went on many dates with beautiful women there. There are many interesting things to do on the site too like quizzes and games.