Facebook is the new Philip Morris!
I fail to understand your argument that you will only need to target people not companies is fundamentally flawed. Your premise is that self-regulation is the only effective way and society cannot do anything to put pressure on the companies to mend their ways is questionable.
If a society is serious about social media addiction as a problem, you will need to target your interventions at people, AND at curbing the companies producing and spreading and hooking people to their contents.
Sadly neither you nor I know what will work beyond that warnings are useless unless accompanied by additional measures. Make accessing Facebook/Twitter/Snapchat expensive after a fixed period of browsing, tax heavily: say you are allowed x amounts of free hour of browsing Facebook no matter who or how you pay for your Internet access. Who knows what will work?