Two years ago I said that Facebook will be dead by 2020 [1] and that line made me check the current situation, we are right on track and linear extrapolation still hits zero in mid 2020 [2]. As mentioned in the original thread, don't get fooled by the normalized numbers, the search traffic for Facebook is enormous and chances are good that any search term you consider popular will be indistinguishable from the horizontal axis if you add it for comparison. But »facebook« is already down from ten to four times »porn«. And yes, the trend can and possibly is at least partly due to other factors, some are also discussed in the original thread.
[1] https://news.ycombinator.com/item?id=11442935
[2] https://trends.google.com/trends/explore?date=all&q=facebook