I've been trying dating sites for years. The best explanation I have for when they work is whether or not someone has a zillion other ways to meet people. People who do find that dating sites work better.
People I have met via eight years on dating sites: 16 year old, borderline psychotic college professor, bi librarian who hadn't been with a guy in a dozen years and subsequently dated another chick, gay librarian (and both of those are different from the one I met from speed dating!), chick who said her mother died the same night she started talking to me and then wanted to hot chat anyway.
So anyway, you really lucked out if it worked for you. I can tell stories of what happens when it doesn't.
I never felt like I had a zillion ways to meet women. I'm sociable, but bars and sports are not at all on my interest level. I did join photography groups and got outside to do meetups from meetup.com. That was a nice way to meet people in general, but I never did it as an actual dating resource. I was just keeping options open and finding people with similar interests.
I also tried a few of the free dating sites for a while. You were the one who mentioned okcupid to me before I had ever heard of it and I did try that site for a while. I dated a girl from okc for a about 6-8 months before we split up. I didn't like the way things were going and ended it. I dated another girl who I knew from high school (who I oddly reunited from a brief time being on MySpace) for also about 6 months before she went a little crazy and we broke things off. Then I took a break for a little bit and tried the online thing again. It seemed like there was nothing for a while and then there was 3-4 people that were all interesting and holding conversations all at the same time.
One girl on okc was about 6 years younger than me at the time (24y/o) another was a few years older than me (38 y/o) and then there was Nora who is one year younger than me (29 y/o). The girl who was 24 was very nice, but I felt like she was still in the younger crowd (rightfully so) and I would feel like the old weird guy, even though I'm not that old. I met her online first before the next two girls. Then I met Nora and the 38 y/o girl both online at about the same time. She was on okc, Nora was on PoF. I went on one date with the girl on okc roughly at the same time I started having good conversations with Nora. The girl on okc was nice, but I found her to be ultra competitive on everything which was a turnoff. Plus there was a distance thing with her, she was a little over 100 miles away. I ended things after the one date with her and dated Nora ever since. Nora was also much closer to me being roughly 20 miles away. The online dating scene was interesting but it went on for a while. There were times I didn't really do much or participate because I felt like there wasn't anyone I was that interested in and then near the end when I met Nora, it seemed to blow up with multiple people being interested all at the same time. Normally that sounds wonderful and awesome, but keeping track and writing/chatting with 4 girls at the same time was not easy.
Coincidentally, I was just planning my first date with Nora the time I went out to San Francisco for a vmware conference and I met David for the first time. Nora and I went on our first date shortly after I returned home from that trip. I did luck out with the online dating thing with Nora. I know there are lots of horror stories of things that don't work out, but this one did for me. All of her family is great and I love her mom like if she was my second mom. Sorry to hear you've not had much success with the online dating scene. I have another friend who has also tried using it for a while without success. He's also been trying several of the different paid sites. I only tried Match.com for the first 15 or 30 days they give you for free, but canceled after that. The other paid sites I know little to nothing about them.