I'm surprised I haven't seen anyone mention this addition to the ToS:
"Except if you opt-out or for … 'Excluded Disputes' …, you agree that all disputes between you and Instagram (whether or not such dispute involves a third party) with regard to your relationship with Instagram, including without limitation disputes related to these Terms of Use, your use of the Service, and/or rights of privacy and/or publicity, will be resolved by binding, individual arbitration …"
So if you fail to opt out of binding arbitration and they do go ahead and use your likeness in ads without your permission, you don't get to sue.
"Except if you opt-out or for … 'Excluded Disputes' …, you agree that all disputes between you and Instagram (whether or not such dispute involves a third party) with regard to your relationship with Instagram, including without limitation disputes related to these Terms of Use, your use of the Service, and/or rights of privacy and/or publicity, will be resolved by binding, individual arbitration …"
So if you fail to opt out of binding arbitration and they do go ahead and use your likeness in ads without your permission, you don't get to sue.
Classy.