From the self harm angle: if you can provide meaningful evidence that use of [social media] results in negatives that far outweigh the positives in virtually every case (this is true of smoking), then this tax makes sense. This isn't easy, though, as millions of people have benefited massively from social media.

As for societal harm, I imagine going forward we'll settle on some countermeasure against entities like Facebook. If that's a per-byte transmission tax, then so be it.

Perhaps a tax that scales with the frequency that someone uses social media? If you rarely use it, the marginal tax is near zero. As your usage increases, the marginal tax applies increasing friction.