lots of user X advertising = profit
valuation typically comes from:
profit X magic multiplier
where your magic multiplier is functionally a premium based on twitter's future profit potential
they're a publicly traded company, and last I remember they report their earnings publicly to shareholders
according to http://www.cnbc.com/id/102573751
their earnings were
$436 million
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I've noticed more and more ads on twitter: in their android widget, on their apps, on their site.
I'm constantly getting emails asking me to use a twitter ads coupon or read about new ads features.