They presumably join for the same reasons that we do--to keep in touch with friends they've only met once and family they've never met, broadcast intimate details of their lives, and post cat videos. But while we young people are aware of the awkward, uncomfortable, weird and terrible things that Facebook seems to magnetically attract, old people simply do not. Thus, Facebook is a treasure trove of awkward old-person moments.
Another misdemeanor old people commit on Facebook is taking everything seriously. EVERYTHING. For some reason, they don't recognize that people of younger generations enjoy sarcasm, irony, and quoting obscure songs.
And of course, there's the subtle but ever-present mistake old people make on Facebook: revealing their confusion about Facebook... on Facebook.
NOTE: All Facebook statuses and such were created by me. Any resemblance to the stuff your grandma puts on Facebook is coincidental, but kind of awesome.