Facebook does give each thread a permanent url, e.g. https://www.facebook.com/Chris.V.Snyder/posts/10202102325020368 for Chris Lyspooner's post "I have just been asked to sign and post a petition to the government to stop chemtrails ....." You get these urls by clicking on the "Notifications" globe icon and clicking on particular notifications. However, this list is cumbersome to scroll through if you're trying to find a particular old thread.
Really, this is similar to the problem that arises with Yahoo Groups, which only lets you search through a few hundred messages at a time, rather than the entire archives of a group. It detracts from the ability to make efficient use of institutional memory. Part of the point of using computers rather than paper for our communications is so that the archives can be easily searched; this takes away from that. Facebook is in some respects worse than paper because Facebook is always burying content that, if it were on paper, you could choose to keep easily accessible.
Wikis allow the creation of talk page archives with permanent URLs and section links. Each edit creates a permanent link, assuming the whole revision isn't deleted. The most insidious aspect of Facebook is that only a little bit of a feed is viewable at a time; to view more, you have to incrementally scroll down, which is time-consuming. Even private messages work this way. It is designed to make it hard to look into the past.
You could save those URLs of old conversations. But will you think to do it, before it becomes too late to easily find them? It's not like Facebook has a Special:Contributions page like MediaWiki's, that would let you find stuff based on your own or others' contributions.
It's wonderful that Facebook can be a venue for starting revolutions, but what record will we have of the specific conversations that produced those uprisings? History is made with the help of Facebook, but