Nice follow up of the discussion initiated on why people should geo annotate a place. Some more threads here and here. I would argue that people will do that because they want to keep contact within their small group of peers, to collaboratively solving tasks, to catching up, to be connected in a different way.

Of course, every one of these activities is influenced by our egocentrism, our need to show off our personality and our ideas. Like blogs.

Other reasons for using such kind of communication can be represented by solving collaboratively tasks problems. I.E., I have to buy food but I am lazy, Mike is coming to my place but I can see he is going to be in the grocery in an hour. Therefore I attach a note to the grocery for him, asking him to buy me some stuff.

This is the kind of approach suggested here: The Neighbor Community Project.