I just spent an hour and change reading old LJ posts from people I know now, but didn't really know when they last updated their LiveJournal.
Back in the day, I picked reading strangers' journals
as one of my 150 interests. There's something unique to the culture and style of LiveJournal that got people to write about their lives in a really engaging way. Sometimes strangers journals are full of major life events and insight. Sometimes they're full of the annals of an ordinary life. And since LiveJournal's design rewards people with long attention spans, the latter journal can be just as engrossing as the former.
A series of writings in personal voice gives me a stronger sense of someone's life and personality than hundreds of photos or a mishmash of minor commentary on other stuff on the Internet. Brevity may be the soul of wit, but paragraphs are the heart of expression. Maybe LiveJournal seems like it's in decline because people don't use real keyboards to browse the Internet any more, and writing something interesting is a total pain in the ass on a phone.