Is this one of those things where they site near a wind farm and tick a box on a form that says they want to buy green power but in reality the actual electrons that enter the data center are "from the grid" and not actually exclusively produced from renewable sources?
Whats worse is that after you buy those electrons from the power companies and use them in your process, you give them back for free. Imagine the overall cost savings you could have if someone developed a market for used electrons and you could sell what you didn't need. I bet that its those power companies that are suppressing that idea so that they can hold on to all those $$$.
But to answer your question. If the Green power company pours X amount of electrons into the grid, and the FB pays that company $$ to pull X electrons from the grid and uses them to power their data center, then what is the problem if they also mix with electrons from other sources? What you are proposing (dedicated onsite, pure green power stations for each enterprise) would be cost prohibitive and stupid for a number of reasons - such as not utilizing scales of economy in the power generation, having to colocate the data centers at the site of the renewable generation, and finally having zero redundancy in their power supplies.