USCF: 1370
Chesscom - 1646
FIDE(assuming this is what you mean by ELO): unrated
First, the best work I have seen in this space is done by guys at ChessGoals.com. ChessDojo also has done some work on converting all of the different platforms to fit their training program.
If your question is "What is the difference in the treatment of the same game between platforms?" You can play USCF rated tournaments online at Chess.com and compare the results (points gained or lost) between the two rating reports. There are several good clubs that host USCF rated tournaments (of course you need a USCF ID).
I have played USFC rated games/tournaments on these clubs:
All of them do a great job. Hope this helps!
It depends from person to person. There are rating convertors online to answer these questions between chess.com, irl, lichess, etc.
It can vary, and I think it's totally different skill sets. I'm around 2000 USCF but I don't think that has much bearing on how high my blitz/bullet is. I see myself playing differently between games where I have 90+ minutes on the clock versus 1 minute. Totally different set of nerves too. But as far as math goes, I'd say generally rapid rating is about 200-300 points above USCF rating up until maybe 2200+ rapid.
I Can do the math. post ELO, Chess.com Rating And USCF rating
mine
USCF:1158
Chess.com:700
ELO: N/A