Longs control the supply of "real" shares in the market - why do you think STMP can sit for 30+ minutes and not trade a share? We don't like to sell our shares - simple enough.
The shorts control price. They can line up opposite of each other and sell the stock down all day long taking a penny or two less with each consecutive sale with their trading buddies (kinda like Wall Street's version of "potty-pals") taking turns covering and shorting it until they get the price down enough to trigger FEAR in Longs and get them to sell. BUT - and again it's a big one - they gotta cover at some point - so in reality all the shorts are really doing is building in FUTURE DEMAND - and they hate it when that point is made.
So - what's the moral of the story? Don't buy this stock as an investor if you can't ride out the volatility. If you trade - good luck with that - you'll need it as the stock price goes where the mm and his buddies the shorts want it to go, but the Supply (of "real", ie. not borrowed) stays relatively static with share buy-backs offsetting management's option exercising dilution for the most part.
The current situation stacks up like this. A big short or a few big shorts are underwater - have been for sometime - unless a lot of those shares were shorted during the bubble years, but from what I've seen shorty is too much of a hyper-active-trader to hold a position that long...
Shares Short as of 04/30/2013 equaled 671,866. Down from 04/15/2013 with shares shorted equal to 753,531 - or a 10.84% drop in 2 weeks. In other words mr. hyper-active-trading-I-don't-really-exist-shorty managed to cover a whopping 81,665 shares over 11 trading days or about 7,424 shares per day on average.
AT THAT RATE OF COVERING IT WILL TAKE shorty OVER 101 (THAT'S ONE HUNDRED AND ONE!) trading days to cover all of those shares. What kind of earnings and other fundamental news will STMP generate in the next 101 trading days?