The rise of Facebook must surely end soon
Not many companies can boast more than 1 billion customers. Even fewer consumer goods companies and tech giants have accumulated double that number.
And none have done so as quickly as Facebook. Last week, Mark Zuckerberg announced that 2b people now log on to his social network every month.
Facebook would never call them customers of course - Zuckerberg preferred to use the term "community" - but nevertheless, it is an astonishing feat.
It took eight years between Facebook being founded and it reaching its first billion; the second billion took less than five. Currently more than 5m new members join every week.
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Facebook's reach is truly extraordinary. Estimates currently peg the number of people around the world with internet access at around 3.5b. Subtract the 750m or so in China, where Facebook is banned (ignore the small minority who bypass the country's Great Firewall) and you're left with a mere 800m people who are online but don't have a Facebook account.
Facebook's growth on the stock market since it went public in 2012 has been similarly impressive (shares have quadrupled in a little over five years), as it repeatedly defied expectations that its membership expansion must slow down.
Yet Facebook's run does have to end at some point. The laws of large numbers can be delayed but not ignored. Globally, Facebook has two-thirds of internet users outside of China.
Zuckerberg cannot rely on more of the world merely coming online for growth. That number is rising, but more slowly than the rate at which Facebook itself is growing, even with the company's focus on making the app work on cheap phones and spotty internet connections.
Facebook is well aware of this and has embarked on deals with mobile operators to provide subsidised data services and made investments in developing its own infrastructure.
Its attempts to give the emerging world free internet have stumbled, however. It turns out that telecoms operators are not very keen on Silicon Valley oligopolists getting a free ride on the back of their infrastructure investments.
Governments have been even less keen to allow Facebook's restricted version of the internet to take hold. It only allows access to a handful of services including Facebook itself.
It looks inevitable that the user growth that has made Facebook one of the world's most valuable companies has to slow down, soon enough if not imminently.
Facebook investors are betting on its momentum continuing. But when user growth starts to slow down, how much each user is worth will become more important.
Of Facebook's 2b monthly users, around two-thirds log in every day, a key metric for measuring how "sticky" social networks are.
This figure has held remarkably steady for two years, suggesting that even if more people are using Facebook, the rate of engagement is not improving.
Other runes are less encouraging. Independent figures have shown that even if users are logging in just as often, many of them have been posting less in the last couple of years.
Many complain that the social network has become dominated by brands and viral content, and lacking the updates from friends and family that made it so attractive in the first place.
This is no more in evidence than Facebook Stories, a feature the company cloned from Snapchat in which photos and videos disappear after 24 hours.
Stories have been a graveyard since the feature launched in March. In a telling echo of Google's attempts to launch a social network to rival Facebook, the most active users seem to be Facebook employees.
The company has inconsistently provided investors with other indicators of how hooked its users really are.
In April last year it said people spend 50 minutes a day across Facebook itself, its Messenger app and the Instagram photo network, but has been silent on the matter since. Research from eMarketer suggests the time spent on Facebook itself - the most lucrative of the three properties - will stall this year.
It has not updated investors on other crucial metrics, such as how much time its users spend watching videos, in over a year, a further indication that engagement may be flatlining.
Facebook still has a lot going for it. It bought Instagram for a bargain and it is growing spectacularly. It is succeeding in earning more from each of its users, as advertisers siphon money away from traditional media and into the company's targeted ad network.
In the last year average revenue per user has risen from US$3.32 to US$4.23. In the US, where digital advertising is more mature, it is $17.
But already being part of a digital advertising duopoly with Google, there is a limit to how much this can continue.
- The Telegraph, London