There isn't some natural law that states "Twitter is with $12bn." It'a valued as such in expectation of future profits. Massive future profits. If it can't sustainably materialise those profits, it loses its valuation (and some of its investors lose their shirts).
Is it really a multi-billion dollars company, though? Twitter has never made money, and it's not clear they ever will make money. A company that can't make money isn't worth more than its assets minus debt.