Inflammatory Mode On: Why in the fuck would even want to opt-in to such a service? If it's private WiFi, it's likely to be at my home or my workplace, and in either case I absolutely do not ever want to share that over fucking Fuckbook, Twatter or whatever stupid lame-ass soshial neshworking crap site becomes the next biggest and greatest.
Rational Mode On: Now let's imagine that my organization has a private WiFi hotspot available for employees and a few others. I do not ever want to have those keys shared outside that group, nor should I have to change MY network with an "_optout" on the end of an SSID. I would consider that a breach of security. Sure, I'll probably be able to disable Windows devices that are domain members via GPO, but if they're not actually devices belonging to the organization, or "Pro" versions of Windows where it even knows what the hell Active Directory is, then MY network is being compromised by this service.
This is just a plain bad idea, whether you're being reasonable or inflammatory.