I sense that strangers are asking you to go on Skype. You're right to ask the question - that's really not proper behaviour (especially if their English or Russian is poor and they're not interested in learning Vietnamese). Feel free to delete the message as soon as you read it. There are a lot of good members here on italki, and as time goes by, you'll find some "real" people to talk to. If strangers message you with creepy comments, then block them and report them to italki (there's a "report" link on each profile, under the member's photo).

Skype is a useful way for the teacher and student to converse. It's fairly easy to install, is free, and is well known around the world. There are other ways, such as Google Hangouts, but for most people, Skype is the popular way to connect.

Yes, It is useful and much easier. anyway, surely everyone had his purposes, i can't guess all of those before contact with them