I can't say I knowingly have led anyone to Christ online, although I've tried many times. And I have learned some things:
When I started out I went toe-to-toe with resident atheists on some boards. In hindsight, this wasn't the best approach. Backed up with some apologetic knowledge, I felt my viewpoint was solid, but they were never going to accept any of it (except maybe for correcting a few misconceptions in regards to some surrounding facts). And they always seemed to have far more time than I had - and scrolling through forum posts it looked like I was outnumbered 10 posts to 1.
I've had more success in telling others my own testimony. God has moved powerfully in my life, and that of my family. When writing from the heart people are much more open and a lot less resistant. And I've found this to be true in my real-life as well. In fact in real-life even the most rabid atheists I know are usually quite respectful when I tell them my story - especially if they know me personally and realize I'm not a crack-pot.
Also, never underestimate the power of prayer. I can also vouch for the effectiveness of fasting and prayer. My girlfriend was originally pagan, and now she's more or less agnostic - and we've been clashing on religion somewhat. So I've spent some time in prayer about it - and also fasted for a day. Anyway last Thursday she said to me, "The most weird thing happened today. Two different online bookstores and also my book club all sent me the recommendation for the same book - and it's a religious book. I never buy religious books. And the book isn't even out until the end of November - why would they send this to me now?". When I looked at it, it was a book called "The Boy Who Met Jesus: Segatashya of Kibeho", about a pagan boy who had Jesus appear to him and give him a mission of living a Holy life for Him. I knew immediately it was more than coincidence.