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.

Awesome! Just a question - how did you go about finding interested candidates who wanted to participate and take a Skype call?

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.