That's an interesting case. I might make some wild guesses based on what I see:
1. vk ports features from fb 3 to 6 months after they appear (share-widget, like-widget, connect-widget, possible friends)
2. vk's got a comprehensive search engine with lots of filters by age, city, gender, current status (married, single, in active search etc.). Lots of people I know use vk for dating. I might be missing something, but I didn't find such a thing for facebook.
3. vk's got native music and video upload. By native I mean that you don't use any third party application/service to do this. Everything's hosted on vk's servers. That's the reason copyright owners sued vk several times (RIAA even blacklisted the guys). However, they [copyright holders] lost in all the court cases.
4. VK offers cheap contextual ads. While not exactly a killer feature, it is very nifty for business, -- they only charge per-click and if you get the targeting and ad text right, you'll get more visitors than you need.
5. Another random guess is that it's easier for a German than for a Russian to migrate to English service.