No there is no threshold because Facebook is in no conceivable sense comparable to public land. The reason the public square is public is because it's not fungible, there's only one center of the town.
You can be on a dozen social networks, or on none, and hitting enter and going somewhere on the internet takes (almost) the same time regardless of where you go,in other words Facebook is trivially replaceable the same way it replaced MySpace, regardless of how large it is. The question is whether Facebook exercises market power, not whether it is large, which is the exact same flawed argument that people use in regards to Google.
What actually has public character is the physical infrastructure of the net, maybe the ISP level, but not digital products, at least none I have seen yet.
You can be on a dozen social networks, or on none, and hitting enter and going somewhere on the internet takes (almost) the same time regardless of where you go,in other words Facebook is trivially replaceable the same way it replaced MySpace, regardless of how large it is. The question is whether Facebook exercises market power, not whether it is large, which is the exact same flawed argument that people use in regards to Google.
What actually has public character is the physical infrastructure of the net, maybe the ISP level, but not digital products, at least none I have seen yet.