Right lane is for cruising.
Any lanes to the left are for overtaking, and you move back to the right after passing.
Of course it depends on how heavy the traffic is, but if you're not willing to move faster than the cars on your right, CHANGE LANES.
People driving in the middle lane when there's nobody on the right are a pet peeve of mine.
Edit: pasting this below, since people seem to be unaware of the actual rules.
Edit2: This is UK, flip left with right for almost every other country.
The highway code, the official rules for road use in the UK, reads:
(General Rules, 160) "keep to the left, unless road signs or markings indicate otherwise. The exceptions are when you want to overtake, turn right or pass parked vehicles or pedestrians in the road"
(Motorway lane disciple, rule 264): "Keep in the left lane unless overtaking. If you are overtaking, you should return to the left lane when it is safe to do so (see also Rules 267 and 268)."
Many other EU countries have the same rules. I wouldn't know about America.