Because social media is simply one of the vast amount of websites on the internet, it's unfair to tax it alone. Either you tax the whole internet comsumption (e.g. every 5GB or something like that) or you don't.

Imagine taxes specific to laundering machines that only apply when washing underwear, but not for any other clothes, it makes no sense.

Also, the govt. snooping on what people access / banning use of VPNs raises privacy concerns (and even censorship ones).