With an $18b market cap today, Twitter would be an “easy” acquisition for any of the tech giants including Facebook, Google, etc.
And at a $2b+ revenue run rate, it would not be hugely expensive.
Facebook is already worth $300 billion and buying Twitter at a 20% premium would be cheaper than the effective price of WhatsApp at closing.
But, probably, there is a sense (fair or not) that Twitter is in part the past, not the future.