A company that doesn't make money buys another company that doesn't make money. The Silicon Valley economy exists entirely on its own with no relation to the "real world".
What makes you think that Twitter isn't profitable, or at least has some incoming cash flow that isn't pumped in by VC or the secondary market?
1) No ads. I don't see any now. "But, they can add advertisements later!" No they can't. Adding advertisements to Twitter will destroy their brand and everything that makes them "cool."
2) No clear picture on their profits and revenue. Profitable companies or companies with a solid revenue model have nothing to hide. There's no reason to hide your money situation if you're successful or have a successful strategy. Twitter doesn't. Their strategy relies on selling to the VC community, not selling to users.
3) Buying up competitors before you're posting profits means twitter's only asset (users) are hard to maintain control of.