From one year after the date of the parent comment Eliezer expects that Google+ will not appear on Hacker News (except as a mistake, an obituary, an "It failed, I told you so!", etc). The probability Eliezer assigned to his prediction being correct: 55%.
I asked for the number with the intent of betting against him. At that figure I am not quite comfortable staking money. That is, I am not sufficiently confident that google+ has a significantly more than 45% chance of taking off. (Because I think that 45% chance of still being significant in a year is a good place to be, given the potential payoffs!)