The dopamine system was discovered by two neuroscientist Arvid Carlsson and Niles-Ake Hillarp in 1958. At that time the system was believed to control and assist in critical brain functions like moving, sleeping, mood, motivation, attention and most importantly pleasure. Since then new research has come out to show that dopamine causes seeking behavior. This seeking behavior increases our level of arousal and goal directed behavior. Unlike an opioid response which is associated with liking, this newly discovered dopamine function is more associated with wanting and is much stronger then the opioid system.
This can explain why while searching for something on the internet curiosity strikes and before you know it you have 10 tabs open on 10 different topics. Our dopamine system activates and causes us to seek more information and if something along the way catches out attention (also dopamine related) then we will impulsively research that specific topic until we are satisfied. We also feel satisfied when we do things like check twitter, emails, and facebook among many other things. When we don't check these things and know or think we might have a new message we can get nervous and anxious until our dopamine system kicks in, pushes us to seek this new information and once this information is uncovered delivers pleasure as well as reinforcing this seeking behavior.
The dopamine system is also keen on unpredictability, it thrives on it. Most of us have gadgets that delivery notifications. Often you can feel a little rush if a notification window unexpectedly pops up or you hear that signature "ding" assigned to your new messages. This is the dopamine stimulating you. This same idea is why gambling, specifically, slot machines are so lucrative. Lastly this research has shown with relation to messages, that short texts or tweets (up to 140 characters long) produce the strongest dopamine rush compared to a long read such as a lengthy email or news article. With all these new discoveries it's no wonder why social networking and new instant gratification gadgets like smart phones and laptops are so successful. The human brain wants more random unpredictability and easily accessed information, which is exactly what these gadgets offer.