Your net is too wide
An internet average would be totally useless. That's like asking how your income compares to the global average. That wouldn't give you much insight into your quality of life in San Francisco, for instance.
Analyze the competition
What you want is a reference against a competitive set or some other target set. The type of business or content you're in will significantly influence your NPS.
NPS is a reasonable high-level indicator of how users feel about a given experience. But the number itself has to be viewed relative to the historical trend for that site and the space in which it operates.
I've worked on sites that were hovering in the mid to high 80s, but actually had more UX issues than others sitting down in the 60s. The product, business model, demographics, and other factors are all represented in that number.
NPS is a really broad indicator
As an indicator of overall sentiment, NPS will include many factors that UX can not impact. That's okay, you can still influence the total with UX improvement. But don't expect UX alone to ever get you to 100. For the record, I've never seen a 100 ;-)