Since they're using it internally to make facebook work, I'd say it's a core product and a lot of engineering efforts are being expended to make the company run better.
It just also happens to be something you can sell to others too.
Since you consider it a core product, what do you estimate the portion of Facebook's revenue comes from this core product in relation to the billions they receive elsewhere.
It just also happens to be something you can sell to others too.