[throwaway5250]

There are very few jobs that allow one to be productive most of the day (and almost invariably they are low-paying). This is indeed shocking to discover in your first jobs after college, etc.

Rather than goofing on FB, I'd look for ways to learn new skills (that ideally somehow might help your employer in the future). Or, alternatively, to find side projects that might aid the organization.

Ultimately it's your manager's job to determine whether you're sufficiently productive. Not yours.