That said, a company is worth whatever someone is willing to pay for it. If Twitter (or anyone) can create the perception that their company is worth x, and then sell it for x, then it really was worth x.
This implies transaction takes place in an efficient market - i.e. bidders have full information about the thing they're buying. Tech start-ups market is quite far from being an efficient market, as we see proven time and time again.
Absent that, it's still true that company X is worth Y to person Z if Y is willing to buy Z at valuation Y. The fact that someone would do the transaction at a different valuation doesn't change that fact.
"worth" is actually complicated and it's somewhat silly to think that it can be collapsed into a single number.