Senator Bernard Sanders just denounced US income inequality for over 8 hours on the floor of the Senate.

This was not a filibuster. He was not blocking any action.

Am I to infer that the Senate which just doesn't have time to repeal DADT or ratify START or etc etc isn't doing anything on Fridays ?

This is all Senator Reid's responsibility. He could tell Republicans that they can deliberately slow the Senate by filibustering bills they support etc. But then they will work on Fridays just as their constituents do.

Evidently that would be too radical a break with the norms and traditions of the Senate.