More specifically, the Internet is a channel or instrumentality of interstate commerce, both of which the US Supreme Court has held fall under the power of Congress to regulate interstate commerce.

That implies that the internet's primary purpose is commerce, which I wouldn't say is the case.

It is great for advertising goods, just as a newspaper or magazine is. That doesn't make a newspaper just a medium for commerce.

I think your definition of commerce is much narrower than the accepted legal definition. Communications is commerce, hence why the FCC can regulate radio transmissions.

Is commerce taken to mean any action that crosses state boundaries, then? Could you point me to a source that backs that up?

Probably the most recent authority outlining the limits of the commerce clause would be the case of United States v. Lopez (1995).

Search for "channels of commerce." Congress can regulate things such as roadways, waterways, etc, where commerce flows.

But that case has nothing to do with communications, and congress lost the case.

The case lays out the current understanding of Congress's power over commerce:

"Consistent with this structure, we have identified three broad categories of activity that Congress may regulate under its commerce power... First, Congress may regulate the use of the channels of interstate commerce... Second, Congress is empowered to regulate and protect the instrumentalities of interstate commerce, or persons or things in interstate commerce, even though the threat may come only from intrastate activities... Finally, Congress' commerce authority includes the power to regulate those activities having a substantial relation to interstate commerce, i.e., those activities that substantially affect interstate commerce."

Thanks for clarifying, that does seem to imply pretty widespread authority. What if commerce is only an incidental use of something, though? Commerce is related somehow to a huge number of things which couldn't be considered primarily commercial. There has to be some reasonable limit placed on these things. The internet, radio, newspapers, and so on are not used primarily for commerce.

The power to regulate commerce encompasses the mediums of commerce. This issue was litigated in 1824 and decided in favor of the broad reading of the commerce power espoused by the federalist camp of framers.

Neither is a navigable river, but the federal government regulates then under the commerce power and has virtually since inception.