Yeah, I have several friends who will post/share things on their wall to "find later." Yeah, there's pretty much no way you're going to find that later unless you manually scroll through pages and pages of old posts. Finding stuff on FB is darned near impossible, with their "search" being woefully inadequate.
It's worse than you thought. Weeks ago, my mother was looking for a conversation with someone who had passed away. I found that there's some sort of threshold problem snatching older posts (or certain categories of user conversations) out of userland.
First: I may be in the dark as a non-member, but neither Facebook's GUI and search tools nor my mother as a user have clear ideas of post categories. To find a keyword and look for the proper search option, it was a pain having to grill her just to find if she had posted the conversation on someone else's "wall", vs. her own, vs. under a picture, vs. a private message chain, vs. a Live chat. I even asked if this happened over Yahoo mail. Armed with a rare keyword I found that the search results had irrelevant posts plus one brief part of the conversation... most (or all) of the search results lacked links back to the posts.
My mother's conversation was from replies to a Status update she made. I think FB makes that data join her wall. River pagination is becoming a thing of the past, to the detriment of users who only have pointers to "now" and "the beginning of time" instead of a clean "x days ago" or "january, february" list option. Since everything happened months ago, scrolling down her wall / river without a filter tool is impractical.
Second: I tried to beat Facebook's search by rolling my own. I mentioned post categories earlier because FB itself uses them to split up member data in your downloadable account activity data. We downloaded hers. Browser searches through the five or six relevant raw html files did NOT show the keyword there. Various greps over the *entire* folder archive also failed. This happened even though older posts were available than the one Facebooks search tool had confirmed to exist.
The whole point is that giving you GUI search results of something you can't see in your full activity archive is proof that they selectively snatch data from your halds. We still know Facebook keeps all the data for whatever purposes the advertisers and research^W experiment teams need it. I recall that EU had laws forcing FB to make a physical CD available with the same archive zip data we can get. The discrepancies should be looked into, but FB is a free service anyway. I know shadow profiles get around some of our options, but the data we were looking for was user-initiated.