I don't know of any laws that this violates. You should handle it by telling her that your actions are not illegal.
This answer is intended as informational only, and does not constitute legal advice or form an attorney-client relationship between us.
I have litigated this very issue in Will County several times. You will not be able to avoid court if your ex decides to file a motion. Consult with a lawyer and be prepared to go to court and respond if your son's mother remains adamant that the page should be taken down. There are arguments on both sides of this issue. If you have not yet attended mediation, suggest that you do so. Perhaps the 2 of you can come to come happy medium. If not, this could get expensive for both of you. Keep in mind that it's what is best for your son and should not be about either parent.
She CAN take legal action. This is an issue for the custodial parent to address. If you have joint custody, deal with it jointly (it sounds like you two cannot do that).
The better thing for you to do, here, is drop it to preserve the peace. If you want to "post pics so family members can see his growing progress and leave comments," do it on your own page.
Sign up to receive a 3-part series of useful information and advice about child custody law.