The most important factor is that number 3: music and video uploads. You just didn't present it as it is. Facebook also has an option to upload video. But the real difference is that VK became practically the alternative for torrents: all the movies and all the music is there. On their servers. Free to search, watch and enjoy. Facebook doesn't have and will never have it.

So if you have all your friend on VK AND you have all the entertainment stuff there - why switch? Unless Facebook comes with some interesting strategies to pull the users, the status quo will remain as is. And from what I hear and see, FB is putting a significant effort into this. It'll be interesting to watch in the nearest 1-2 years.