This would be ideal. The network effect is strong. As Barrin92's response to my comment stated, anyone can make an alternative site. But all of the value comes from the large networks of people on these sites.
I would prefer if such a "public square" site weren't for profit, if I'm going to be honest. Not sure how well that would work, but profit motives introduce too many flaws for such a necessary part of society. OTOH, so do influences from other means...