Google Trends can't reliably tell you anything useful about a site's growth or activity. I'm not denying it correlates with your specific app, but that doesn't mean it will work for everything, and it clearly works very poorly for popular services.
Remember when everyone was sharing youtube videos. Or when you would sit and endless watch. I feel like that faded about 45%.
Google dropping makes sense. I visit less sites and the search results are not that great for discovery. Most sites I visit by typing a letter in the browser and auto completing.
Facebooks earnings raising 20% makes sense as more people are advertising on it. I notice 71% less people less original activity.. photos/status updates more groups/ads filling the gap. I spent 71% less time on and reduced my daily visits.
We're always chasing yesterday's tail. I bet twitter's numbers are down.