How hard is it to meet people backpacking?

I’m going for a month or so to Europe this summer by myself… how hard is it to meet people backpacking through europe, staying in hostels? I’m an average outgoing guy and usually doesn’t have too much trouble meeting people – I would just like to be able to go to bars and clubs and such and don’t really want to do that by myself. Any suggestions on where to meet people? Thanks

I’m traveling to London, Paris, Barcelona, and Rome.