What's actually happened is that fewer people are using search engines as their app launcher, because they are using the smartphone home screen instead.
Isn't that a plot of people who search Google for the term "google"? If so, I imagine it's something people who are familiar with computers and the internet do not do often.
No comparable web properties have the dropoff that Facebook has (twitter, eBay, craigslist, even Yahoo which seems to be fading faster into irrelevance by the day).