How the "public" uses the service is irrelevant; the legal regulations are all based on what the service is.
Social media is fundamentally different from ISPs and other utilities- there are some protections and restrictions under section 230 (for now) but number of customers does not a utility make.
No, use of public space (spectrum, running wires to homes) drives the regulation. If the wireless spectrum was unlimited, and power, tv, internet and phone lines didnt need to be run on public land to get to private homes, they wouldn't face anything like the regulation (nor be granted monopolies) like they are now.
Social media is fundamentally different from ISPs and other utilities- there are some protections and restrictions under section 230 (for now) but number of customers does not a utility make.