The number of nodes on the network has no direct correlation to demand. This is because it is possible to use Bitcoin from many different types of client. Only a small number of Bitcoin users still run a full node due to the overhead involved in running a 150+ Gb database. It's not something that is possible from a mobile phone for example.
Is there no way of tracking thin clients as nodes, or do these not count as nodes to the network? I get that there would be no way of doing this for something like an exchange that "funnels" many people, but wouldn't a hypothetical thin client on a phone still count as a node?
No, a node is something that actively participates in the running of the network. Thin clients connect to a node, so you would need them to make the data of how connections they have available, which they don't.