Geo fenced Wikipedia exists already, as a number of apps are available which offer regional maps with localized Wikipedia additions, see for example https://wiki.openstreetmap.org/wiki/OsmAnd
Twitter started as an internal service for existing Odeo employees, so an unfair comparison if you're talking about cold, starting a social network. Twitter didn't start cold.
Like what is the highest rated/longest Wikipedia article in the area.
Or maybe what's the 3 top radio stations and a link to them.
There is plenty of local content that Google does not surface