A couple thoughts:
People do pay to get into a certain building to meet other single people (I'm thinking about nightclubs and bars that have cover charges here).
Also, your point about people mostly meeting partners through friends is valid but the REAL question is does it happen this way becasue that's the 'best' way to meet somebody or becasue it's currently one of the biggest ways humans meet new people? In other words, does it work this way becasue this way works best, or does it work this way because it just happens to provide the most chances?
That being said, your assessment of the problem is solid. It's an idea worth honing in on. The one thing you'd want to be able to hang onto from the "walled garden" sites is the fact that everyone there has expressed interest and availability for a dating relationship. This lowers the risk for those who do the asking since you are less likely to get rebuffed in the "EW, I'm dating already, thank you very much creepo!" sort of way.