I have found this reviews site to contain the most detailed information so far and from my web experiences. Dating sites for the most part do not give out paid members information just usually only the total number of members, both free and paid. I believe in one article I read that Match.com had over a million subscribers of their total 15 million membership. That is a lot but the members are spread around so it may not work for you where you live. With paid dating sites not giving out the number of paid members information is similar to the free dating site not giving out their actual user numbers. I know Plenty of Fish have a lot of members but how many actually visit the site every couple of days or actually reply to their mail? It would be nice to see this information on a per region basis.
How effective a site is I find usually depends on who you talk to. I argue with my friends all the time on which dating site works. I think all the major dating sites work but it just depends on the person and on his or her profile, their location, the amount of time they put into it and a bit of luck that there are a few compatible actively participating people on the site with you at the same time.