I think this is partly because it gives the illusion of contact with people without any reality to it. Because the audience is so broad, one tends to share only the most superficial things: those who are prepared to share everything with the world use it as a one-way broadcast medium in a way which makes me feel uncomfortable.
I can't quite pin down why I feel cramped and uncomfortable using facebook and not, say Twitter, which I find much more appealing and open. I mostly use the @oldiesclub twitter account but I may try to use my own @cycas account a little more in future.
I gave up on Google plus, I've still got one but found it too noisy and distracting. I know I can filter, but it just seemed too fiddly.
This is the first post that I have ever used the 'morose' mood option on. Strangely, this cheered me up, so the mood is now a bit of a lie :-D