> If people find a life partner they drop off the site, and you constantly have to find new users to replace them ... As a result, some of the less-scrupulous sites purposely make it less likely that you will see people who seem like great matches

I've often heard this sentiment, but I'm very skeptical. Firstly, most users in my experience seem to prefer casual relationships even if that's not what their profile indicates. Dating apps are also one of the highest vectors for cheating today, and one man even confessed to me on our second date that he has a wife in Rajasthan.

But I think the fact that the major apps' user bases all grow over time indicates they have a naturally healthy replacement rate. People turn 18 every day and desire a partner. People break up every day, etc.