I think doing this comparison of social apps is so much fun I'll keep doing it on a bi-monthly basis. The data is from Facebook, where "monthly users" is based on the number of connections to Facebook.
The data is for nov 2011 ---> jan 2012 ---> mar 2012.
News in the "other" category: Pinterest doubled in last two months! Instagram almost, as well. Gowall and Oink have been killed.
News in the Swedish category: Spotify added 4,5 million users in 2 months. Soundcloud and Wrapp still growing strongly.
Previous updates: here and here.