There exist many utilities that themselves are built on top of other utilities.
The reason something should be an utility has to do with competition and how much power customers. The primary argument in favor of having Internet providers be an utility is that creating an competing Internet is not really an realistic concept, and the power customers of ISP has in their customer-provider relation is close to nothing. Utility laws exist to solve this specific situation in the market, and making every single company an utility would not be very efficient way to make use of those laws.
Let's examine two cases for an ISP utility. They both lead to the same conclusion without turning everything downstream of ISPs into utilities.
1. The utility of an ISP is that of an ad hoc communication platform. Here we must ask why an ISP is an ad hoc communication platform and Facebook is not. Either both are or neither is.
2. The utility of an ISP is derived from providing access to Facebook. Again it is either both or none.
Ah see, I disagree that an ISP is in itself an adhoc communications system.
An ISP hooks up the wires, lays and maintains the infrastructure for web connections for which you can build communications on but are not in themselves sufficient as communications. You can also build more than just communications on ISP infrastructure.
As examples, you watch Netflix on the infrastructure of an ISP but Netflix is not communications and you may bank on your bank's website on an ISP and that is not communications those are movie watching and banking respectfully.
I could continue to provide examples, but my point being is an ISP is not in itself sufficient for communications to be established and that you can establish more than communications on an ISP and that is why I don't agree.
The reason something should be an utility has to do with competition and how much power customers. The primary argument in favor of having Internet providers be an utility is that creating an competing Internet is not really an realistic concept, and the power customers of ISP has in their customer-provider relation is close to nothing. Utility laws exist to solve this specific situation in the market, and making every single company an utility would not be very efficient way to make use of those laws.