Is it obligatory to return a greeting in writing in forums if it was written, or is it sufficient to return it in one's heart, and is it obligatory to initiate or return a greeting when commenting on a writer's words?

If the writer intends by his greeting of peace everyone who enters the website or converses with him, then you must return his greeting in kind, in writing, or with something better than it, both verbally and in writing. It is not sufficient to respond only in the heart, due to Allah's saying: {And when you are greeted with a greeting, greet [in return] with one better than it or return it [in a like manner]}.