Twitter have $4.4bn of current assets, half a billion of current liabilities and $1.5bn of long-term debt. That leaves $2.4bn unencumbered (of which $900MM are trade receivables and prepaid expenses). Meanwhile, they're losing half a billion a year.

Google is saying no to paying $18bn (just under $15bn in enterprise value) for that. I have a tough time valuing it north of $5 or 6bn EV, and that's assuming $500MM in earnings within 6 years (and break-even in fewer than 3).