Recent articles around Facebook advertising opportunities – such as sponsored Ads etc. – have surfaced well-worn stories of Facebook as a “Google-killer,” perhaps due to a limited understanding of the differences in Facebook and Google users’ intent, and misunderstanding of core index data.
Searches within Facebook are generally around a user’s current network, previous associations, or one’s own interests / brand affinity; i.e. looking up a friend, old school mate, known brand page or interest areas.
Google searches generally have an intent around key information, pricing (comparisons), or facts; note Google Knowledge Graph  aims to address at least part of this intent.
Secondly, each platform’s core dataset is radically different. Google follows links and other signals to discover content, indexes that content and then catalogs, categorizes, connects and compartmentalizes key data of each webpage / website, to address user queries with relevant content that connects to user desired intent. (Even Google makes mistakes in interpreting intent, but it’s a robust system that mostly works).
Facebook on the other hand, isn’t as vested in crawling the web, instead relying on users to share, like or provide information that builds their core dataset. Jessica Lee  and I had lively discussions around Social Search Changing the Search Paradigm  underscoring The Search Agency’s “Search Everywhere Optimization” SEO philosophy, that brands need to be where users search, and that is increasingly not happening on the major search engines.
Users flock to where they get the most relevant results. Platforms such as Google and Facebook can leverage both active and passive search to build revenue opportunities. Key here is that users drive the largest opportunity through active searches, enabling platforms to connect their user’s search intent to advertiser offerings. Google has pretty much nailed it as a business model, and continues to iterate to provide a great search experience, valuable content and well-curated results. Facebook on the other hand, doesn’t (currently) offer a similar breadth of search results’ quality that would motivate Google users to switch (and many advertisers to invest as a core strategy.)
Whether one is an earned or paid search practitioner, alternative search venues need to be reviewed, assessed and tested. In a holistic approach to search marketing Google is a non-negotiable necessity in any initial strategy. Facebook is taking a few sword swipes at the Google giant, but as of now only inflicting some minor flesh wounds.