quite often, in such company sales as well as in stocks/real estate and many other forms of investments, the sale price is a function of future earning potential. Of course, the future earning potential is not often the sole factor, but probably the single most dominant factor in the equation.

Even current profits/revenues are seen as an indicator of future health (along with being an indicator of current health) of the company.

$3 bil might not be much in the not too distant future. New markets that were not practically existent before or are far from being mature, are coming up online - read, india, china, brazil, south east asia and what not. Their online advertising industries have hardly developed, not that those in US or Europe have really matured. There's lot more to come.....

New models like twitter have shown quite a good capability in penetrating not only the developed markets, but also the developing markets. Of course, it still has a long way to go.