Facebook wants a single platform to provide the Web through. If they built a search engine, it would only search Facebook.
Example of a search query URL. Verified pages appear to get more weight. https://www.facebook.com/search/top/?q=president
Despite many real (though also some exaggerated) counter-examples, Facebook does have features to protect privacy.
One of those things is you generally can't get very useful results from search unless you're friends with someone, or a friend of a friend (depending on the user's privacy settings). You can't see general trends, or even search for every person with a given first and last name in an area, for example.
It used to be more open, but they've heavily restricted the breadth of data returned from searches within the past few years.
On top of which, Graph Search is still disabled inexplicably in many regions, for reasons which were never explained.
Ditto. I have seen this as a crippling deficiency in the Facebook platform. Not being able to search properly in my own posts or in the groups I'm a member of really sucks. For all the engineering prowess, open sourced tools, etc., shown by Facebook, the lack of a working search makes the company seem incompetent from the top down.
Sometime ago I started storing important information (like others links, my own comments, etc.) outside of Facebook where I can find them easily. I also started a Facebook page and added Notes into it to make it easier to document, find and share things. It seems ridiculous that I'd have to do this just to have access to information, but that's been the sad state for years.