Not according to
this chart from last april. Looks like South America has the highest Spaces usage per month... putting Spaces at exactly 1 million hours more than the number two social networking site in Africa, the continent where the popularity of social networking is, of course, huge.
Most market share results I can find put Spaces at 0.2%. I guess it depends on your definition of 'high', of course.
That said, I've never even heard of most of the sites on that chart. Bebo, Skyblog, Hi5, Cyworld, Orkut, GaiaOnline and Mixi? Kind of depressing that even most of those usually have a higher market share than Spaces.