I would put them in three groups, from most vulnerable to least: Netflix, Facebook/Google, Amazon/Apple.
– Netflix is greatly overvalued even for a growth company and has a single revenue source with real competitive headwinds.
– Facebook and Google are single revenue source companies with significant regulatory risk and are somewhat overvalued by growth company metrics. If any risk to that single revenue source manifests, it will kill the valuation.
– Amazon and Apple have diversified revenue sources and are relatively inexpensive by the usual growth valuation metrics. (Microsoft goes here but the stock is expensive.)
85-90% of their revenue is advertising, a single revenue source. They have never had another major revenue source in the history of the company, and not for lack of trying to diversify.
For all practical purposes, this qualifies as “single source”. Apple receives about as much revenue directly from Google as Google receives from all non-advertising sources combined.