As Peter Thiel wrote in Zero to One, both small and giant companies will try to creatively redefine the definition of the market they are addressing, in the first case claiming that they absolutely own a small market segment of questionable relevance and in the second case claiming that they are small fish in a much bigger market than the one where they are enjoying a monopoly. So the results will be determined not by some objective standard but by relative political power of these companies and their adversaries.

As for durability, I personally fail to see how google and facebook are not durable monopolies - the network effects are so strong. In google's case it is even more than that - it is almost impossible to have a product of comparable quality without the users and the information about their behaviour, which is impossible to obtain because everyone uses google. Gone are the days when good search rankings could be inferred from the link graph of the web.