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Jennifer Golbeck lists some of the FOAF-enabled social setups: Social Networks on the Semantic Web

Total Number of Members: 8,220,800

That's presumably including some duplicate people, but still quite a figure… I don't know if they are all exposed enough to call them members of the Social Network of the Semantic Web. But to that I think you could also add all the bloggers that are using tools (like WordPress etc etc) that have XFN support, it's explicit data available as FOAF via GRDDL (see XFN on the GRDDL).