The reason telling a person directly may be muttar is because it is l’toeles. Posting it on the Internet, on the other hand, and it certainly will be seen by people who it isn’t toeles to let know. Thus the issur of L”H remains.
Similar, in a way, by shidduchim where you are allowed to tell someone information l’toeles ONLY if you know he will not spread the information. Otherwise you cannot tell him, even if it is legitimately l’toeles for him, if he will then tell others who it isn’t relevant to.