I suggest you do a little bit more research on what constitutes defamation. There are 3 basic elements for establishing a case in defamation and "在社會上具一定知名度" is definitely NOT one of them.
另外,誹謗有分民事及刑事,但要構成刑事,必須牽涉惡意發佈及公衆利益,一般情況,要證明發佈言論者是刻意有難度,所以原訴人多以民事索償。可能現時法例太寛鬆,加上追查網上發佈者有困難,才令你誤會誹謗只是民事罪行,縱容了網上惡意發佈虛假言論的人。我們有誹謗條例架!有時間不妨去看看。