Google's revenue today is six times as large as it was a decade ago. And I expect its revenue in a decade to be at least 3 times as large as it is today. The reason is that many of the causes of past growth will continue for the foreseeable future:
1. The people who are connected to the internet will continue to become richer - in particular booming poor countries like India and Indonesia
2. The amount of time people spend on the internet will continue to grow
3. The amount of advertising Google can show people per hour will continue to grow. For example, Google can show many more ads on YouTube and still be less saturated than the old network TV model. Another example: The Economist app currently shows untargeted ads copied from their print magazine - but in 10 years these ads will probably come from Google.
4. Google can continue to improve its algorithm for placing of ads. Google knows so much about me yet I'm amazed how stupidly untargeted its ads sometimes are. Eg. if my photo stream doesn't include pictures of cats, don't show me ads for cat food!
5. Google can continue to collect more personal data. Right now it doesn't know when I last purchased toothpaste, but it might in 10 years from now.
6. The number of people connected to the internet will continue to grow (although of course at a slower rate than in the past)
[Disclaimer: I own shares in Google and Facebook]