make a sharp turn called a pivot or whether we should persevere along our current path. - Eric Ries

Startups are different: too much budget is as harmful as too little — as countless dot-com failures can attest — and startups are extremely sensitive to midcourse budgetary changes.

Metcalfe’s law: the value of a network as a whole is proportional to the square of the number of participants. In other words, the more people in the network, the more valuable the network.

the paid engine of growth is powered by a feedback loop. Each customer pays a certain amount of money for the product over his or her “lifetime” as a customer. Once variable costs are deducted, this usually is called the customer lifetime value (LTV). This revenue can be invested in growth by buying advertising.