[Animats]

Sell to whom? They're already publicly held. Selling means someone has to buy up all the stock, which requires about $15 billion in cash.

[andruby]

Not necessarily in cash.

With a leveraged buyout, you can go as low as 20% in cash and finance the rest with loans you put on Twitter's balance after the acquisition.

Or the acquiring company can issuing new shares and using those to pay Twitter shareholders.

[Animats]

Twitter doesn't have the balance sheet for a leveraged buyout. No hard assets and money-losing.

Google or Facebook could buy Twitter for cash, but it's not clear it would be a good investment. It would be better to wait for Twitter to fail and then pick up the pieces at a huge discount.