The simple fact is that Bethel just doesn't care. Once people leave they are gone.
People who are still known as JW's are faced with nosey elders, Ministerial Servants and their wives who like to make other people's business their own. But I know from my days both at Bethel and as an Elder there is no campaign to search out people online aside from individual efforts.
The Internet was just coming of age when I was at bethel and there was HUGE paranoia about it. I was one of the first bethelites to have my own PC and Internet account. I had many friends at Watchtower Farms and within Canada IT and even back then there was no concentrated effort to seek out JW's online to track their activity.
Again, we would all like to think we matter to Bethel but we don't. They are so convinced about the rightness of their ways that they don't spend a moment's thought about our activity online unless it is somehow involving a legal or potentially legal matter.
The only thing they actively pursue online is copyright infringement and they are equally as nasty with believers as they are with unbelievers/opposers.