Facebook's mobile apps have to use the Facebook API, which is notoriously terrible. Facebook have no reason to devote resources to this, as they don't want third party implementations dragging people away from their ad revenue.

Similarly, their mobile app is capable of delivering far less ads than the web page, so they'd rather people sat on that at their desk rather than browsing on their phone.

In relation to lack of decent search and other obviously lacking elements: They're always going to devote far more resources to things which directly or indirectly improve advertising (graph search, friend finding, increasing numbers of likes) rather than things that enhance user experience (since user experience is far more tied to network size rather than feature set, compared to other markets)