Doesn't matter. No matter what your churn is, if you currently have a thousand times more members then your competitors it will be drastically easier to attract new members. In other words, your competitors have a much higher customer acquisition cost. It's still a big barrier.
The churn is so high on dating sites that everybody is constantly re-acquiring customers. One test for anti-trust is also how they think the market would respond to a price increase.
Yes, you need a critical mass of people, but what this means can be very different for different people. Some dating sites have a warm body approach where you probably need lots of people because the set of matches is terrible.
However if you had a dating site for swedish people living in miami who only wanted to date swedish people even having 40 people on there might be plenty.