If Google disappeared this weekend, we'd wake up Monday morning and our Android phones would still work, a few people would miss Play and the worst part would be listening to people complain about Bing search and everyone else's maps.
If Exxon disappeared I'd be in line at the pump filling up gas cans in addition to the tank and on my way to stockpile canned goods, potable water. and buckshot.
With your reasoning, for example, the private prison companies would be valued incorrectly high (as do the companies that make the green lights for traffic intersections, the doors to stores, the electric power company, the wireless/phone/cable company, any company with any kind of monopoly, etc). What you're forgetting is to weigh for the difficulty to replace the company with another (that in some cases may already exist and only needs to move into the now vacant market).
This is a pretty bad analogy, and even at that, it's not completely thought through. Google is responsible for a lot of the infrastructure on the modern web. Gmail, ad networks, google apps, data centers, etc... If they disappeared this weekend it would be chaos.
If exxon mobile disappeared, we might see an oil shock similar to the one in the 1970's, but more likely, due to strategic reserves, we would see a gradual increase in prices, followed by a ton of investment into increasing production capacity by the other oil companies.
If phones still work then it's only fair to say that the pumps, stations, etc still work and someone would take it over and provide the same services, disruption would be fairly short term.
If Exxon disappeared I'd be in line at the pump filling up gas cans in addition to the tank and on my way to stockpile canned goods, potable water. and buckshot.