[levitratestimon]

I don't know what the ulama have said on this issue, but I would personally refrain from talking about the religion at non-Islamic forums for a few reasons:

1. People are less restrained when there is anonymity so the non-Muslims would freely hurl insults at Allah , Rasoolullah , Islam in general, Muslims at large, and directly at you.
2. On forums where the majority is not Muslim, there is usually a gang mentality, especially if the forum has a majority belonging to a particular religion (e.g. Catholics). Just the fact that someone is not a member of the religion adopted by the forum majority irks them and they try to coordinate attacks and offensive posts directed at you and aspects of Islam.
3. The best way to do da'wah online is not by posting on non-Muslim forums because people see proselytizing online in a negative light. Responding for the defense of Islam is a different matter, though.