The IRS starts to look at social media in most cases when the taxpayer owns an online storefront. Such sales transactions don’t get reported automatically to the IRS, or at least not consistently.
So if the IRS sees your tax return reports a lower-than-expected low income, but at the same time you possess an online storefront, you could be in for some hurt if you don’t report the sales.
However, the IRS won’t just start snooping anyone’s social media account. In all likelihood, you’d need to be on the IRS’ radar already for them start looking. It can, though, open up the audit for greater scrutiny and can get worse at that point.