I know it's a month before Valentines day buy the stores are already full of heart shaped candy. OkCupid is one of the largest dating sites. The video above from TED talks explains their method for coming up with a % match. The logic is pretty simple. It's based on how well the questions that both you and your potential match answer. The more questions you both answer, the more information you both have about each other. The site is free for a basic membership. I have used it and so far haven't had any luck but others have.
eHarmony is another enormously popular site. This site used to be pay only but, perhaps in response to new sites like OkCupid, allows limited access for free but those who pay have better access. Above their CEO Greg Waldorf (not Neil Clark Warren who is in their commercials) discusses their formula for matching individuals. They use a personality inventory along with the stated criteria given by individuals to match. Also in response to OkCupid they have added questions on various topics which may or may not improve their matching process. Waldorf makes a bold claim that 2% of all marriages are due to their site. I don't know how they know this except they might track their matches and look for self report or newspaper notices of their marriages.
There are of course plenty of other dating sites now. Match.com was the original site and has been around for almost 20 years. Christian Mingle, a dating site for farmers, one for singles over 50, one for black singles and Plenty of Fish also come to mind. The bottom line is that online dating is now big business and consumers expect results. The boom can be partly due to the harsh economic times as Waldorf states. Being married does provide some measure of economic security that being single often does not and people are still looking.