Nielsen reports that two-thirds of the world’s Internet population belongs to a social network, though the by-country stats show a little more variation: only half of Switzerland’s and Germany’s residents are on social networks, while Brazil has 80% of its population social networking (Orkut FTW!). In Brazil, one in every four minutes online is spent on a social networking site. (In the UK, it’s one of every six minutes, in the US, it’s one of every sixteen.)
On the other hand, the smaller markets are seeing the biggest growth—Germany was the fastest growing country in social network membership last year.
Facebook is the big winner in this race, however. Worldwide, the most popular social network saw a 566% increase in time spent on site last year, coming to total 20.5 billion minutes on site. (!) They also premiered a German-language version in March 2008, which has somewhat with the increased popularity of social networking in that country (although FB still lags behind more established sites).
Nielsen also documents the demographic shift to older users on social networks worldwide. The full report (PDF) includes some good suggestions for advertisers and publishers, as well as data on country- and network-specific uses.