Earlier quoted context omitted.
You write like everyone who posts to Facebook or Twitter expects you to read everything they post. I don't think anyone posts with that expectation in mind.
I understand that isn't the case, but they obviously want as much attention for each update as possible or else they wouldn't bother and regardless of their expectations, it's still more stuff that every other person has to sift through.
I think, though, that audience targeting is the weak point of Facebook and Twitter right now. I think many times people post something with a few people in mind or at least a subset of their social graph. For example, when I post technical articles that probably are interesting and relevant to my technical peers, my HS friends and family will see them also. I don't think you can blame users for doing this or chalk it up to vanity. Their posts are probably relevant and interesting, but just for a subset of their graph.
A Googler made an interesting presentation describing this problem: http://www.slideshare.net/padday/the-real-life-social-networ...