It provides us almost immediate positive reinforcements and that encourages us to keep coming back.  When you click a link, you are given something to look at, experience, and play with.

The actions are immediate – depending on who’s on.  If you tweet, you can get retweeted.  If you update your status, it can get “liked” within seconds.  If you post a blog, you can get hits and comments – which all make us all feel good.  It’s is immediate.  It feels good.  It returns to you the effort that you put into it.  You come back because you want to.