Outside of everyday conversations, you still need to get your thoughts out. Thoughts that you can't simply talk over sympathetic ears, thoughts that you have no qualm in getting it out to the general public in a way which you would rather not and choose not to in casual conversations.
Hence the proliferation of social networks - a rather misleading name, if you were to ask me. Because it does seems that the bulk of 'social activities' are rather monologues in which you hope others were to join in.
But anyway, that's only part of the point. The other point to all this monologuing about is when you realize how your inner thoughts aren't necessarily profound and significant when you were to compare with others. Well, perhaps that is to be expected, since we're all humans after all, and we operate on similar basic principles...
I would just point out that what we view as a profound observations, rants, philosophy, discussion etc - our thoughts - once we get them outside, we would no sooner realize how it isn't as profound as we make it out to be in the first place. That is, of course, if we take a step back and realize this. And by me saying this, I'm not saying it's useless or waste of time, no. I'm merely pointing out how some of us give our own thoughts too much value.
We feel like we said something significant when the rest of the world looks on, engrossed in each own's thoughts.
There is of course no problem in getting your thoughts out. Hey, at least it articulate your points clearly, and you can see what exactly goes on in your mind. But of course, perhaps we should be selective about it? Self expression, think of it that way.