What is the ruling on a wife participating in mixed Facebook pages under the pretext of da‘wah, when her husband has forbidden it because he considers it to be intermingling (ikhtilat)?

A woman is permitted to enter mixed-gender online platforms for da'wah (inviting to Islam) if she is safe from fitna (temptation/strife). However, it is preferable to avoid them. The fields of da'wah for women are vast and are not limited to these participations. If her husband forbids her from doing so, she is obligated to obey him.