No way will I or most people I know be happy with a significant resource (like a Semantic Web) being owned or subject to the whims of one or a few people or organizations. Open Access controversies gives a hint of how much unhappiness will result if Facebook, Google, Microsoft, or any one entity looks to have a lock on such a vital web resource.
Here the matter is even worse due to the network effect: if most of your friends are using facebook's social tagging service, you'd have to be connected to it one way or the other.
yahoo has more email account than google.
The web we have now is not 100% federated. Our access and ability to publish on the web is controlled by a few large companies with some regulation by the government. Even if the Semantic Web takes off, I can't imagine that most people will be consuming that data autonomously. On the contrary, there will likely only be a few significant players be they browser producers or otherwise converting that information to be relevant to users, thus it won't really be the federated utopia so often espoused by the Semantic Web pundits. But hey, maybe I'm a pessimist. It's not to say we shouldn't care about privacy and about how big conglomerates with their own agendas use the data we give them, it's just that it's naive to think that there is some purely free form of the web yet to be discovered.