Social networks like Facebook, Twitter, really need to find other kinds of monetisation than subscription fees, if only because they become exponentially more useful the more people use them. Restricting the number of users by charging them wouldn't help them.
If social networks aren't able to find ways to successfully bring in revenue using advertising they deserve to go out of business. One of the big challenges in advertising is not knowing whose eyeballs are seeing your ads. For TV they may have a crude demographic of who's watching ... but it's not very granular. Social networks are the exact opposite of that. No only do the users of these networks willingly provide details about themselves down to the most granular level, but they also provide a linkage to their friends that are doing the very same thing. It's the perfect combination of information for advertisers to use in order to serve relevant advertising.
