Because Google use how much your pages content is being discussed / posted within social networks to help them frame such mathematical outcomes of importance to whether your pages content is quality or not. Social network referrals is a huge part of the equation. Massive actually. Google don't determine your pages importance by its content, but how much it is being discussed, linked, so forth. Googles algorithm has simply turned into reliance as a single huge social network infrastructure.
Google make statements that building back links can hurt you, yet Google is still so easy to manipulate with just a handful of authoritative back links into your site / page, giving it massive boost. Google can't determine a pages importance any other way, thus until mathematics can be turned into AI, Google can be heavily manipulated. Social networks are just the current easiest method to do it.
My below image shows my slow decline from Panda... nearly 3 months ago I started pumping up social networking referrals, and whilst that line is just above the base line in that image due to the huge spike early last year (thousands), what you're seeing is a social activity of around 30 referrals a day suddenly jump to hundreds a day (just coming off the line), and you can see my resulting traffic increase. Slowly it's coming back... and we aren't talking hundreds here, but thousands per day.
I expect that within 6 - 9 more months, as a result of this little trial... I will have returned to my prior years peak traffic and then hopefully continue where it left off, continually climbing again.
The entire time I was still pumping out to Facebook and Twitter, and it continued to decline. It wasn't until I got out to a whole bunch of other networks that turned it around, especially Google+, as Google give their community referrals more, similar to Reddit, Pocket and Sodahead, all of which are premium social partners for Google to use. The more you get into Reddit and other partners to pump out your links, let alone all social networks, the better you will do with the current algorithm.