I really think Starbucks became popular in part because of all the celebrities shot by papparazi drinking Starbucks.
Britney used to be ALWAYS photographed drinking a frap and you can always see the Olson twins carrying Starbucks and loads of other celebs.
Carrying a Starbucks became a sort of "fashion accessory", like a purse. I am totally serious.
Some people think they "look cool" when they are carrying Starbucks.
This is the REAL reason why Starbucks got so big :
A lot of people follow their favourite stars everyday on all sorts of forums and blogs and everytime they see a celeb carrying a Starbucks it reinforces it that it's cool. Starbucks is basically getting free advertising.
Not many people can afford a $2,000 purse like Lindsay Lohan or a pair of $700 shoes like all the celebs but everyone can afford some Starbucks...
If you have a product you want to become popular, if you can get someone like Lindsay Lohan get caught by the papparazi using or holding it or wearing it, the next day thousands of people will rush out to buy it... Seriously.