Mark Zuckerberg's 6' stone wall around his $100 million Kauai property has blocked out the Pacific Ocean to neighbors ... some of whom are livid.

The Facebook founder bought the 750-acre estate in 2014 and now wants to shore it up, but a 5'8" neighbor for one is furious, because now she can't see water.

Some neighbors claim they've tried reaching out to Zuckerberg, to no avail.

TMZ contacted the zoning department in the area and we were told no one has lodged a formal complaint and there is no investigation.