The best social networking site is going to depend on what you intend to do on that social site. If you're trying to build a business and connect with your target audience, then you'll want to go to the social site where your targeted audience is easily reached.

For some people it may be Facebook for others it may be LinkedIn or another social site. PR of a site doesn't really mean a whole lot these days in terms of marketing. You want to judge a site my the engagement of its members, how easily people can be reached, and most importantly, if your target market is hanging out on that site.