In a perfectly beautiful capitalist society-- that is, a small community-- I would create a better site and put him out of business. But due to the sheer number of users he has now, the switching cost (for each user) is almost assuredly too high for me to compete with him barring some great publicity or a large sum of money. I might still do it, though, just for kicks.
I don't think it's a good idea to compare his company to YC-funded/inspired sites, because he didn't set out to make a successful company; he made a site to learn a language, and people gravitated toward it over time. He got lucky by entering his market accidentally at the right time, with the right features.
Some people voted me down, I assume for saying that I "disrespect" him. But I really do. Any businessman who doesn't believe in advancing society with the power and responsibility that comes with business ownership deserves no respect nor success. But the fact is that whether or not someone deserves something, they sometimes get it; and he did.
But in the grand scheme of things, looking at plentyoffish.com makes me want to vomit, and that's why I highly doubt it will remain successful indefinitely. The only problem with overtaking its lead is that it's a user-to-user business, and competing with a company like that is hard for someone new like me, because at first there is zero benefit for signing up due to the lack of users. The first few thousand users might not even get anything. So you end up with an alarmingly high switching cost.
In the war between beautiful design and achieving benefit from the product, the latter almost always wins. At least in the short term.