I've been messing about with some online sarging over the last 2 months or so, but I have to say it's a bit disappointing and nothing compared to real life game.
The main problem with it is that, say you send a girl a message, she can delete the message and you will learn NOTHING from the experience. In real life game, everything is positive - even if your lines don't work to perfection, you can still take from that experience to better your game and maybe it'll work on the next girl.
With online game you have no idea where you are going wrong because you receive no feedback if they're not interested, which kinda ruins and defies the point of the game for me.

I'm out. And I'm going to the pub to see some three dimensional women.

It depends on the site. You can discern some behaviours by experimenting with how you portray yourself. At least with plenty of fish you know they read and deleted your mail and you can mostly tell whether they looked at your profile or not. But it is generally a design flaw on dating sites not give people better feedback. Giving people the option to hide profile viewing is the worst idea ever.

It can be both good and bad. Good if your getting a nice response from a girl, then when u meet her in real life u already have stuff to talk about

Bad cos like u said, they might just not reply to u on facebook chat or msn. In which case, u don't want to keep messaging her cos you devalue urself