The rumour for the $2.5 billion offer came from "a source close to an executive involved in the talks", while the $4 billion offer was leaked by a Twitter shareholder. Both sources say it's likely that Microsoft was behind the second offer, which means that Twitter found itself in the midst of a bidding war between two of the biggest corporations in the US and beyond.
It sounds likely that such informal talks occurred and if true, this means that Twitter spoiled the party. Yet, these astronomical figures are another confirmation that we live in digital times - after all, Verizon bought 22 MHz of spectrum (or polluted air, if you look at it from another angle) for $4.7 bilion in 2008 and invested new $4 billion for its LTE and 3G expansion. No wonder Twitter is offended by the $4 billion offer.