The idea of a company targeting and following individual high school students on instagram does feel a bit "off" to me. If facebook or any other billion dollar public company were doing this, a bit of public uproar would not be out of line, even if it is perfectly legal.
But it sounds like TBH stopped doing this after they were acquired by FB, claiming that the methods were too “scrappy” for a big company. While facebook is of course now ultimately responsible for any actions that TBH took in the past, trying to make a big stink about it after they have already stopped (and using misleading headlines to boot) feels a bit disingenuous.
EDIT: I initially missed the fact that TBH was essentially pretending to be affiliated with the school of the kids they were targeting, by creating a new account per school and saying things like "You’ve been invited to the new app—stay tuned!" (A fairly dumb oversight on my part as that's essentially the whole point of the story). This is indeed hugely deceitful and having re-read the article it changes my opinion entirely. Facebook should be banning companies that pull stunts like this instead of buying them out for millions. I would like to see Facebook issue a statement clarifying their position on these kinds of marketing ploys, update terms of use if necessary to prevent them, and also probably issue an apology.