With just over 10 million unique monthly users, it's not the largest of the dating sites (although it is owned by Match.com), however I do believe it has the best offerings on this list with regards to cost (free, with options to pay for a few fun, yet non-crucial features), number of members, and usability.
More » With just over seven million unique monthly users, e Harmony is the largest dating site focused exclusively on heterosexual, long-term partnerships that (hopefully) lead to marriage.
While I found the site cluttered and confusing to use, they're obviously doing something right with three million unique members visiting every month.
Having said that, I've found a larger-than-average number of folks looking only for something casual here (likely due to the lack of time investment needed to join), and I've yet to hear of any successful relationships come out of the site (where readers tell me they've met The One, or something equally special, on the site).
Unlike paid dating sites which only want you to pay them all the time, Date Me Mate Me will never take your money.