You won't get enough data if you ask them to fill out a survey. If you try a single interview you'll see why. You'll end up with pages of notes and venture into problems you had no idea about. If you had asked them to fill out a survey instead, you would've ended up with just a fraction of the information.
And world wide or not doesn't matter. I did all my interviews over Skype.
Reach out to your network or email your friends to put you in contact with potential users. It should be moderately easy since you're just asking for a free Skype call, you're not asking for money yet.
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.
And world wide or not doesn't matter. I did all my interviews over Skype.