FORTUNE — One of the great things about the Race to a Billion chart above, posted Friday on Horace Dediu’s Asymco blog, is that it suggests that all computing platforms eventually reach a natural limit, where their growth starts to level off.

• PCs running Microsoft (MSFT) Windows were the first reach 1 billion, but because the platform took more than 10 years to get there, the leveling off doesn’t show on this 44-quarter log-scale chart.
• Facebook (FB) was second, reaching 1 billion user accounts roughly 35 quarters after Mark Zuckerberg began signing up Harvard students in 2004.
• Google’s (GOOG) Android platform was third, hitting 1 billion activations in a record 20 quarters.
• Apple’s (AAPL) iOS platform, by Dediu’s estimate, will reach 1 billion units sold sometime next year, roughly 24 quarters after the launch of the first iPhone in 2007.

The only trajectories on Dediu’s chart that don’t yet show any signs of slowing are the number of active iTunes accounts and the number of Apple TVs sold.

LINK: Third to a billion.