These are the top European countries in terms of Facebook users (in brackets, the percentage of their population using Facebook):

UK: 14.9 million (24%)
Turkey: 7.9 (11%)
France: 6.6 (10%)
Italy: 5.6 (9%)
Spain: 2.6 (6%)
Denmark: 1.8 (33%)
Sweden: 1.7 (18%)
Belgium: 1.7 (16%)
Norway: 1.5 (31%)
Germany: 1.3 (2%)
Switzerland: 1.1 (14%)
Greece: 1 (9%)
Israel: 0.9 (12%)
Serbia: 0.6 (6%)
The Netherlands: 0.4 (2%)

Sources:
– Wikipedia por the world population
– Facebook users

What looks quite interesting:
– Germany and The Netherlands have very few users. It is true there is StudiVZ (8 million users) and Hyves (5 million users), but I also think that Germans don’t like social networks so much.
– The top countries are the Scandinavian, so there is a relationship between number of Internet users and number of Facebook users.
– As Matt Dickman reports, Facebook’s population is getting older, as the network gets more popular.