They don't know who you are and you don't know who they are. Neither of you knows what the other person looks like, so your initial attraction is more likely to be based upon personality. You aren't put in the awkward position to meet the person with all your family wondering how it's going to turn out. I feel comfortable placing an advertisement on the net versus a newspaper because for some strange reason, placing an ad on the net seems more casual. A lot more discreet.
People have to really fight their way through to get to the site.
Then they have to again find you via search and select. The Internet reaches more people. I realize that I am limited to people who have web access, but I like the technology and feel like I have more control. I don't have to give out my full name for a person to run a background check on me until I am willing to give that information. I know that the people who respond will have some technological knowledge, which means that learning is somewhat important to them.