The answer is yes and no, and the differences vary from site to site.
First off, when you have an official USCF rating, tournament performance plays a role in your final rating. If you win all rounds of a tournament, you get a bonus to your rating something not available on most Internet severs.
Next, most Internet players have odd and extremely short time controls. While an avid player, I never sat for a tournament with rounds less than G 30,(game in 30). Even that long is extremely rare on say, the yahoo severs. Most tournaments where even longer. The length of the game can affect your skill lvl greatly. The longer I played, the more I could think for each move, the better those moves where.
Finally, most start you off somewhere around 1400-1600. That, in say Uscf, if a really fairly respectable rating, and would be well beyond your typically Yahoo 1600.
I would say to get an accurate USCF rating from something like yahoo, or pogo is nearly impossible. But their way of calculating your rating based on wins and losses is fairly comparable to how official organizations do it.