A more colorable tactic is that you send a notice to their registered agent of addendums to the contract on your website, with possible future updates.
Certified mail to their registered agent is considered “received” by the corporation in most contexts.
Received does not imply "accepted". I can also mail you an agreement in which you point your job payroll to my bank account, but you're not obligated to actually do it, right?
The difference here may be that there's already a contract between a user and the company, so if they include something like "continuing to provide service to my account constitutes acceptance of these terms." in the updated contract, it might fly.
Or not. It could be that all the businesses doing that to their users are just BS'ing something unenforceable.
I prefer to have the sheriff do the civil process to the managers (or managing partner) home address.... that way it freaks them out and gets the best response.
Certified mail to their registered agent is considered “received” by the corporation in most contexts.