Dating sites fascinate me in a 'customer retention' sense- once someone ends up in a relationship, surely they stop using the site altogether? An interesting challenge. I used to work for a housemate matching site that faced similar challenges, but at least apartments are usually based around 12 month leases. If only relationships were...
If you mean that you don't spend money on advertising, I can certainly believe that.
Nope, a very simple one. Just keep the experience mediocre. Not bad enough for them to leave dissatisfied, but not good enough for them to leave satisfied. This will maximize their time on the site. This is largely the result of the monthly subscription model, which creates a fairly twisted set of incentives.
Interestingly, the old school matchmaker model doesn't suffer from this problem. You pay a fee for a successful "match". This creates an incentive for the matchmaker to do her best in finding the right match. This model is out of fashion now, because people prefer to have more control over the process and dating is no longer always about marriage.
Also, most relationships have a shorter duration than most leases, providing a very lucrative repeat business stream.
And mine, personally, would be ok if I explained what I was doing and didn't actually cheat on her, and showed her some actual friends I met. But I agree, it may be a problem.