Do you have a textbook or just free talk? Do you have a scheduled contact or just see who happens to be available on the friends list? Thank you for your sharing!
(The question is obviously not limited to a specific language)
In my opinion, Skype is a powerful tool to practice any language with overseas friends. I use it to talk with people I meet here. Whenever I see a phrase or a verb I didn't know, I write it down on my English notebook and so I can remember it until I get to make it part of my vocabulary.