I am creating a server to monitor the online presence of clients on a webpage.
100 000 persistent connections is not a viable option.
Sending HTTP requests at interval is way more feasible, and writing a dedicated HTTP server is an interesting choice IMO.
Take a look at this question for some orientations.
I know you specifically say .NET, but you should probably take a look at NodeJS, as it does exactly what you're trying to do, but it is server-side JavaScript.