Condemning People on Internet

I am 22 years old and I am from XXXXXX .
In the former Soviet Union Islam is reviving, but many Muslims still do not have much knowledge about religion. There are some Russian-language Islamic web-sites in the Internet and they host discussion rooms and view forums for Internet users. Sometimes a Muslim participant might say something, which is absolutely not right in Islam and he/she might not know it because he/she might lack some knowledge.
My brother in XXXXXX participates in these discussion rooms (view forums) and he asked me to ask you a question, which is as follows:
"... There are some discussion rooms and view forums on the Internet where participants can express their opinions and tell their thoughts on different issues on Islam.
Sometimes a Muslim participant might disagree with certain thoughts of another Muslim participant and criticize his/her opinion or expression. However, sometimes such a critique might touch the participant’s personality and [also] his/her way of belief, i.e.aqeedah.
When somebody is criticizing his/her brother/sister Muslim in such a manner, the latter might not be present in a discussion room and he/she might never come back to see it again. [In my country we have rare access to Internet. This means that somebody might never see the view forum again, hence he/she will never see the answer to his/her thoughts]. However, certainly, this critique becomes known to other participants of the discussion room.
Is such a critique considered backbiting/slander, i.e. ghibah, because of the fact that the one criticizing is not aware of whether the one criticized knows it? Might it be so that a Muslim talks about another Muslim such things, which the latter one would not want to be talked about him/herself behind his/her back.
As you understand the question is important, because I was criticizing some thoughts of some Muslims too.
If it was ghibah, then I will have to make repentance and to ask those Muslims to forgive me and never do it again ..."