знакомы? and кто это? What's the difference and what's the common one?

Question

I want to have more Russian friends from the website and I added them randomly. Some of them, commonly ask me знакомы?. Some are use кто это?. I just want to know what's the common word between this two words are very useful for Russian and what's the difference between them, because there's some Russian or even non-Russian who speak Russian are adding me on Vkontakte and I just want to use this word as to ask them who are they.

Answer 1

As @Artemix has written знакомы? means are we acqauainted? and кто это? is who's there?. The first variant is more polite and can be used in everyday speech as well, e.g. if a stranger contacts to you on the street. The second one is pretty informal and is relevant for use in the Internet when you don't know the person you talk/write to. Originally, before the Internet, кто это? could be asked when you want to know who is knocking your door. So this question implies that you don't see the person. Basically, both phrases can be used in social networks chats with equal meaning. I'd recommend you знакомы?, again, as more polite.