It is a nuisance to the people around you. If they're organizing something and make a FB/VK event for it, someone will have to bother themselves with relaying you all the updates via your preferred communication channels.
In this context he's not wrong. Planning a party (for example) might involve a lot of discussion for time, food etc. It's far more complicated to organize this with everyone and find a common demoninator compared to simply putting people in a group, with everyone giving his input when needed.
Yes. Some of my friends have a VK event for their birthday party that they reuse annually. Much more convenient than chatting with everyone individually or making a group chat (but then everyone needs to be using the same IM service so this problem comes up again).
In the end, if you're not on whichever social media service is popular around you, you're missing out.
We need working federated social media asap.