As long as you were dealt cards and rake was taken on most sites they consider that as you paying rake. Say they took $3 in rake for a full 10 seater table...and everyone was dealt cars...and you folded pre-flop along with 5 other people.

At the end of the hand they take the $3 in rake divided by 10 and treat it like you all paid 30 cents towards rake...therefore you get rakeback on the 30 cents.