I eagerly await the Wikipedia of Data, but I think it would at least have to be an open, non-profit type operation to be successful. I don't think the commercial sector is capable of that degree of cooperation and long-term vision.
There aren't a lot of real APIs. Google has deprecated their APIs for search, Twitter is very restricted with their API and Facebook has a lot of issues with their API.
The irony is that Microsoft is/was more open about their APIs than the web.