My massive decline in use is directly correlated to how annoying they've made it to use.

Eg: You can't have your feed always sort by "Most Recent", it keeps defaulting to "Top Stories", keeping old/stagnant stories at the top of my feed just because they got a few comments, while not showing me the new stuff.

Also, the sheer amount of crap they shove in your face now. "<friend name> Likes TOYOTA!" then a big post by Toyota shoved front and center of my feed, all because that friend Liked that page a few months back.

Also, the annoying way that you can't ignore just a certain type of post from friends (bar a few). You can set them to "Important Updates Only" but then you miss lots of posts you might find interesting. I want to ignore their Likes of memes/pictures but still get their posts of their own content, etc.

There's lots more reasons, but they've basically made the site a pain in the ass to use, so I find myself back to reading sites via RSS and chatting to people directly on Google Talk when I need some "social".