There was an attempt a few years ago to do just this. Remember Imzy? If the answer is "no," you're probably not alone; it shut down after only two years of operation. They didn't have any of the problems that we're discussing here, as far as I remember. (Personally, I suspect its extremely weird and confusing UX design was at least part of what did it in; I found its navigation so baffling I gave up pretty quickly.)
If I remember right, you couldn't see any content on Imzy without creating an account. I assume that instantly turned away over 90% of the users. Then, their design was extremely childish, and it looked like a safe place for kids, rather than a site for adults to socialize. I think they did a really poor job at understanding the market, and I'm not surprised they failed.
How are you going to get any traction when after losing 90% of your visitors to that home page's design you lose 99% of the remainder because nobody is going to join for content that may or may not exist, let alone be interesting. That level of disincentivisation ensures that the site's user base will consist of 90% employees of the site, 5% their friends who signed up and never logged in again, and 5% of randoms that love having a captive audience... until it dies.
