Would you say that by asking ppl to fill in a simple Monkey Survey be sufficient, in case you're looking for a World wide audience?
And world wide or not doesn't matter. I did all my interviews over Skype.
If you're having problem finding people to get on a free Skype call then it's a big red flag. You'll need to reconsider your positioning or your market. Because eventually you'll (presumably) be asking for money and that's even harder.