The passive voice helps shift the emphasis away from who committed the act to what actually happened.
The Senate and the House passed a health care overhaul.
The health care overhaul was passed.
Funds were withdrawn from the account on Saturday.
Alex withdrew funds from the account on Saturday.
“By” is not necessary. If you feel it is, that is probably an indicator that you should be using the active voice. The passive voice works well when we don’t know who committed an act or when stating that is unnecessary. It also works when what happened is more important than who did it.
Economic plan is panned by Geithner.
Geithner pans economic plan.
I would choose the latter. Geithner is the U.S. Treasury secretary, so his opinion is of first importance. The name deserves special attention.