60%?! That a regular user will abstain from an addictive site for about twice its current age? A site about a topic he's obsessed with? I'll take that bet.
(Made my own 5% prediction.)
My reasoning was along the lines of 'well, now he's publicly committed to it and would be ashamed to make a comment or post' and that LW can be something of a habit - and once habits are broken, they're easy to continue to not engage in. (For example, I do not have the habit of smoking, and I suspect I will have ~100% success in continuing to not smoke over the next 5 years.)
Although note I slightly cheat by specifying posts and comments - so he could engage in private messages or voting on comments & posts, and I would not count that as a falsification of the prediction.
http://www.guardian.co.uk/lifeandstyle/2011/jul/08/change-your-life-ugh-fields