The problem is that large-scale use of the Internet for social networks and for organizing meetings in real life is fundamentally incompatible with privacy. It works for small, tight-knit insular groups, but as soon as you expand the scope of the network to include acquaintances and friends of friends you'll eventually find a connection to someone who cares less about privacy than about making a buck.

If we had a sort of "federated" system we'd still have this problem because you might always find yourself federated with someone who just wants to sell the information.

It's a cultural problem within this hyper-aggressive version of Capitalism that we've adopted, that even data about people has value. Until we decide as a culture that this kind of data sale or data use is shameful and unacceptable we'll be in this situation no matter what technical solution we adopt.