Why have the constraint of only a few characters? Let's reverse that.
Let's create a site where you have to write a certain number of characters to post. Urges people to write more, share more. Let's make social sites what they are supposed to do, share things with your social group.
Just like this, let's start writing!
If it breaks or it doesn't work as it is supposed to, shoot me an email and I would love to note it down