When people think of Google, they think search. Google has created this dominance of the internet with consistency and efficient pathways towards relevant information. Their reputation for treating employees well, in addition to leading the wave of new technology in Silicon Valley, has led to people's trust and reliance on the company. Facebook has a similar stronghold, but on a different market. Yes, Twitter has dipped into Facebook's business, but Facebook still remains the giant of social networking and has propelled the industry toward more information-driven pursuits. Companies want to pay Facebook for advertisements, but also for their information. Collecting information on billions of users allows Facebook to sell this data to large companies who can better understand people, and break them down by demographics and psycho-graphics. If Facebook wishes to become a dominate search engine as well, they will need to provide information that Google doesn't offer, or at the very least make the information more accessible than its rival.
Obviously, this graph depicts that Facebook's search queries are indeed on the decline. This is something that is vital to not Facebook's expansion, but it's maintenance as the dominating social network. When people aren't searching, they likely aren't online as long as thus there is less information for companies to snatch up. When there is both less information and less uers available, it is the domino effect of people not being able to reach their friends and/or find out what's new with them because people are no longer publishing their information on the network. Zuckerberg has forecasted this, and it may not be time to hit the panic button yet, but that time may be near.
http://www.wsj.com/articles/facebook-takes-aim-at-search-again-1445552264