I have been involved in only two instances where a poster without faith became a Christian, at least in part, because of his involvement in one or more FR threads. In each instance I did what I could "from afar" to help them find a church in their local area to help them continue on their new journey.
Interestingly enough, however, both of those instances came about, not in any of the so-called Christian threads, but in the Philosophy/Theology threads. Nor did we talk about the Christian faith until they began asking questions about it.
I agree with you that such instances are rare, but I do believe if one is willing to meet people "where they are" all things are possible with God.
Peace.