To turn into a profitable company, valued at (say) a hundred billion dollars, they need to start making more money.
Now, there are two main ways they can do this.
1.) Increase number of users.
2.) Increase average revenue per user
They can attempt to increase their number of users by focussing on countries with low penetration. The problem with this, is that these users are harder to make money off (on average poorer, advertisers less interested in their eyeballs). In fact the article claims that these users are loss-making for Facebook at this time. There is (I guess) little scope for absolute user growth in the US, UK and European markets, in fact, its been suggested that user numbers are tailing off here.
Revenue per user: Given the constraints on users mentioned in the last paragraph it becomes really hard to increase your revenue per user. Especially if they're shifting to mobile which is harder to monetize. Any new users are probably going to harm this metric even more (as they'll tend to be poorer than existing users)
So, if Facebook want to say double their revenues how are they going to go about it?