Does my online chess rating reflect my ELO rating? I play a fair amount of chess online, on a chess site that updates my chess rating based on my performance using, I believe, the ELO system.

I do not play in rated, "real" tournaments that often, and so I do not have an official ELO rating.

Is my online chess rating a good estimate of my "real" rating? I've heard that one's online rating is systematically biased upwards compared with one's "real" rating, is there any truth to this?
That depends a lot on the platform. 

My different online ratings differ by more than 500 points. And there is the additional factor that online chess is not tournament chess. The time control is either faster or looser and you don't have to play at set times. Also your opponents tend to be less serious about it. 

Chess24 has high ratings, ICC has low ratings, your Elo is probably somewhere in between, but to speak of a good estimate would be an exaggeration.