All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
First of all, it is permissible to set up such a webpage if its content is principally permissible in nature. This entails that no promotion of forbidden matters, such as usury or the like, should be made through it. The posts and comments made by participants from both sexes that appear on the page are of no harm provided that they are within the permissible limits of Sharee'ah. The participation of women in any public forum is acceptable if the content is public. What is forbidden is the participation of women in private rooms where the devil may cause temptation. In this case, it is prohibited for fear of that occurring, as it is known it is an obligation to prevent what leads to something forbidden as established by the scholars of Usool (principles of jurisprudence).
However, it is not permissible for you to enable anyone to exploit your account on Facebook to spread pornographic pictures or put links that could lead to forbidden matters, as this is a form of helping in sin. Allaah says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allaah; indeed, Allaah is severe in penalty.} [Quran 5:2]
It is more appropriate for you to write conditions from the start indicating clearly that it is prohibited to post something that displeases Allaah, and that you have the right to remove anything of this kind that is posted. If any such thing is posted, it is obligatory for you to remove it, as you are the person in charge.
Abu Sa’eed Al-Khudri narrated that the Prophet said: “Whoever among you sees an evil, he should change it with his hand, if he is unable he should change it with his tongue and if he is unable, he should change it with his heart and that is the weakest level of faith." [Muslim]. Also, Ibn ‘Abdul Barr reported that there is consensus of all the Muslims that it is an obligation to change evil for whoever is able to do so. One should not refrain from changing evil if it merely means being subjected to criticism, but not significant harm. If one is unable to do [change evil by his hand], then one should change it with his tongue. If again he is unable, one must detest evil with his heart, and he is not required to do more than this. If he detests evil with his heart, he has done what is required of him if it is the only thing which he could do.” [End of quote]
Allaah Knows best.