In popular imagination, Amazon.com ranks alongside Google and Facebook as a tech giant able to make long-term bets by virtue of a founder who controls the company and a cash-spewing core business. In reality, however, Amazon has never spewed that much cash, and its founder has been slowly liquidating his stake.

Since taking the company public in 1997, Amazon chief Jeff Bezos’s shareholdings have dropped to 18.1 percent from 41 percent, largely because he’s been selling shares steadily. If he keeps up the current pace of selling, Amazon will become vulnerable to an activist investor unhappy with the blizzard of costly initiatives Mr. Bezos has undertaken.