Clever rebuttal. ^^
Well, defamation can be hard to pin down - online defamation even harder.
To be specific, to seek any kind of compensation for being a victim of defamation you (generally, this depends on your specific nation, of course) suffer significant financial burden or severe social stress or anxiety as a direct result of the defamation. This would include things like: being fired from your job, being 'asked' to move from your community (apartment building, etc), and such. Hurting your feelings or upsetting you isn't liable.
For the specifics of online defamation the stakes are raised. Quite often online defamation (True online defamation, between individuals whose relationship to one another is purely online) happens between people in two different juristictions, as in, different countries which makes it very very difficult to prosecute as it is already such a wishy-washy topic. In order to prosecute under online defamation you would likely have to suffer financial loss as a result to the defamation. For instance, an online business going under or suffering serious financial loss due to the defamation. Perhaps even an industrious person's online English tutoring classes suddenly emptying would be cause enough? Hard to say!
However, in order for defamation to be an appropriate charge, the offending statement would have to be an untrue statement of fact - not opinion.