Microsoft is a natural monopoly because of the network effects between number of consumers of an OS and number of developers for an OS. What makes Google a natural monopoly?
I wasn't responding to Google being (or not being) a natural monopoly, I was responding to "Google is what I use, and it's what I use because it's Google".
On your point, it could be argued that Google's overall platform dominance might not be attributed to barriers to entry, but rather barriers to exit. While Google offers some of your data back to you in a portable format it's still quite difficult to leave their services, and even when you do they will still be tracking you for advertising. To form their potential natural monopoly in your mind, you can imagine how difficult and inefficient it would be to have multiple different firms tracking you to the same extent that Google does (search, Google+, Gmail, calendar, Google Docs).
