Hmm, this sounds like the old "you shouldn't mind surveillance if you've got nothing to hide" argument.
The IRS doesn't even have the budget to carry out basic audits in which the bulk of the investigational burden is placed on the taxpayer (by means of responding to auto-generated documentation requests). It certainly doesn't have the budget to invest in a high-tech system that could track multiple social media platforms and somehow connect that to financial spending or tax reporting.
Could it happen? Yes. Is it a risk one must take in order to partake in the luxuries of the lifestyle one leads? Yes.
It is not difficult after all to imagine a situation where the worst case scenario of Nike monitoring you is much worse than the worst case scenario of the IRS monitoring you.