Twitter is a hot new IPO and people are jumping on early in the hopes of it rising in price.
This is why people are suggesting going short. It is clearly over valued. The $26 initial valuation was more reasonable, if you assume the market information is wrong, and that Twitter will generate a far higher profit in the future. Your valuation is a bet on the market being wrong and Twitter's mobile potential being stellar.
Of course, your bank account doesn't have much opportunity to grow its business, and Twitter doesn't have FDIC insurance.
More generally, the value of a company is the sum of all its anticipated profits for all time... discounted for inflation, risk, and the time value of money. (The first two of those discounts are obvious; the "time value of money" is just the fact that it's better to have $50 now than an inflation-adjusted $50 in 30 years because you could do something with the money now, like invest in other companies or invest in bonds or invest in re-insulating your basement to save on heating.)
If that growth is unlikely to happen, then it's overvalued. If Twitter becomes quadruple-Facebook and is earning $2.5 billion/year in a few years, it's very undervalued.