To help drive ad sales, Google pays for traffic to their ads. They paid out $2.21 billion, or 24% of their ad revenues in "Traffic Acquisition Costs". That money goes to revenue shares with their AdSense partners and to "distribution partners" -- presumably browser makers, PC OEMs, and mobile OEMs and operators.
This is Google's business. They sell ads alongside "free" content, and they buy additional traffic to make those ads more valuable. http://weblogs.mozillazine.org/asa/archives/2011/12/firefox_...
(I've been quoting that blog post a lot lately.)
My iPhone browser has a google search field, and I am sure that Google pays for that, and that it drives traffic to its ads. It is money to drive traffic, but doesn't mess with Page Ranking.
Afterthought: Also, not only does that mozilla stuff work as traffic acquisition, it's also branding. So there's some other value for Google there.
I do not think social is the key to search. It's not useless, but it's way too overrated IMHO.