But when internet forums count as social media, few can provide a complete list of websites they have engaged with.

I definitely can't recall every one of those I had been on in the last decades.

The law will have to define explicitly what it means to have an active social media account.

I wonder if one could truthfully state "no, I'm not on any social media [at this exact moment, since I'm talking to you and not actively using any social media]" and avoid any potential legal trouble, or if there's some clause or mechanism in whatever law applies here that's designed specifically to address intentionally misleading word choices.