Apps that rely on network effects to be successful need a free model, or an existing base with critical mass. The latter is nice if you have it.

Freemium is a great way to get some users to pay for value created out of the total network effect created by mostly free users.

I don't think Markus Frind was swinging for the fences when he created plentyoffish.com with a free model. I think he realized how hard it was going to be to get enough people on a dating site to make it interesting.