Skype Etiquette?

I have a little button on my website and blog so that clients can contact me directly using Skype. Click, ring, ring, I'd like to enquire about your caricature prices. But I frequently get contact requests or chats from random people, not just in Ireland, but all over the world. I don't necessarily mind (yet!) but it got me thinking: you'd never just pick up the phone and randomly ring someone from the phone book, would you? And isn't VOIP supposed to be computer-based telephony?
Here's my thinking on it- the whole social networking phenomenon (Bebo, Myspace etc) is so ingrained into the culture of computer usage that applications like Skype fall into that bracket by default. By the same token that you can browse and add friends at random on networking sites, Skype users are doing the same thing. Which begs the question, what is considered proper Skype etiquette? I'd be interested to hear your thoughts on it.