Every week, one of the new generation of internet firms seems to attract a sky-high valuation. Zynga, the social-network games company that has tempted millions to grow virtual vegetables in its FarmVille game, has been valued at $9 billion. Profitless Twitter is said to be worth $10 billion. Groupon, vendor of online discounts, rejected a $6 billion offer from Google and is considering a flotation with a potential valuation of $15 billion. Tech-watchers say this is just the start: the real boom will come when Facebook, the head boy of the new dotcom frenzy, goes public, probably next year.