It doesn't take energy, it doesn't take thought, it doesn't take very much at all. And it interrupts the build-up of emotion which might have led to a longer post on somewhere like Livejournal. If you've burnt off your frustration at your fellow passengers on the bus this morning by posting a one sentence burst of anger then why would you bother writing it up in any more detail? And it's always going to be easier to drop off on sentence about how much you love your current TV favourite than it is to write a whole review.
Better (and worse) yet - nobody is going to pick apart your argument, because you aren't making one. Nobody is going to try and dissect your take on your favourite novel, offering you new insights, because Facebook comments don't support that level of interaction. Livejournal offers you a massive area for text input, Facebook gives you a couple of lines. Sure, you _can_ type in as much as you like, but the psychological difference is immense.
And this means that many, many people have moved away from Livejournal. Possibly still reading (I still get a fair few hits per day), but not updating their own journals, not joining in the conversations, not interacting, not being part of a community. And, in its own dysfunctional manner, LJ is a community. But it feels, nowadays, like a dwindling one. One that feels, to me, like it's dropping towards a critical point where the feedack isn't enough to keep people updating at all.
Of course, I've been reading "Livejournal is dead! Any minute now!" posts since 2001, and it's still here (140th most popular site on the internet, according to something Google posted recently). Maybe it's not livejournal, maybe it's the natural turnover as people change, the churn of busy lives pushing people away to do real things in real places, while new people arrive that I have less of a connection to.
But it's something I find myself intermittently missing, that's for sure.