Is it a sin to read Lashon harah online? For example, reading bad reviews about a business. Or even reading reviews about a person? Thanks!
According to Rabba bar Rav Huna (Bava Bathra 39b): כל מילתא דמתאמרא באפי תלתא לית בה משום לישנא בישא - anything that was said in front of three (people) does not involve bad speech.
This leniency is connected to the principle: חברך חברא אית ליה וחברא דחברך חברא אית ליה - your friend has a friend and your friend's friend has a friend (i.e. word spreads and the information is assumed to no longer be private). Since its highly implausible that anything published on the internet is private, this would seem to be grounds for leniency.
Accordingly, the Rambam (Deoth 7:5) rules:
אֶחָד הַמְסַפֵּר בְּלָשׁוֹן הָרַע בִּפְנֵי חֲבֵרוֹ אוֹ שֶׁלֹּא בְּפָנָיו, וְהַמְסַפֵּר דְּבָרִים שֶׁגּוֹרְמִים אִם נִשְׁמְעוּ אִישׁ מִפִּי אִישׁ לְהַזִּיק חֲבֵרוֹ בְּגוּפוֹ אוֹ בְּמָמוֹנוֹ, וַאֲפִלּוּ לְהָצֵר לוֹ אוֹ לְהַפְחִידוֹ, הֲרֵי זֶה לָשׁוֹן הָרַע. וְאִם נֶאֶמְרוּ דְּבָרִים אֵלּוּ בִּפְנֵי שְׁלֹשָׁה כְּבָר נִשְׁמַע הַדָּבָר וְנוֹדַע. וְאִם סִפֵּר הַדָּבָר אֶחָד מִן הַשְּׁלֹשָׁה פַּעַם אַחֶרֶת אֵין בּוֹ מִשּׁוּם לָשׁוֹן הָרַע. וְהוּא שֶׁלֹּא יִתְכַּוֵּן לְהַעֲבִיר הַקּוֹל וּלְגַלּוֹתוֹ יוֹתֵר:
Regardless of whether the slander is spoken in the presence of his fellow, or in his absence, or if the slander spread from mouth to mouth cause harm to his fellow, to his body, or property, even to cause him anguish, or fear to his soul, behold this is the iniquity of an evil tongue. If such evil be spoken in the presence of three persons, the matter is thereby considered public. Thus, if one of the three who heard it repeat it to others no sin of an evil tongue is found therein, provided that in re-telling it he had no intention to spread the rumor and advertise it still more.
Assuming this ruling applies equivalently to the one reporting the gossip as to the one receiving it, there may be mitigating factors (at least according to some opinions) such as whether the subject of the gossip is assumed to mind. See e.g. Rabbi M. L. Halberstadt:
One should not share information about a person with others, even if the information appears online, when it is known that the privacy of this information is important to him.
It is important to note that there is a lot of information online about many people, and they will not necessarily be pleased with the information being publicized. It does not seem that the Halachic limited permit of telling things which were said "B'apei Tlata" (In front of three people) applies to things advertised online, as not any information posted on one site or another turns famous and known to all.
In a case where a person publishes information about himself (not negative) on his own website, he shows his intent that he does not mind people knowing about it and one may share it with others. (See Chafetz Chaim, Laws of Lashon Hara 2, 13 and Be'er Mayim Chaim 27).
See also here, based on Be'er Mayim Chaim 27-28:
There is a difference between monetary or business issues which if a person publicizes himself in front of three people, or in our case on his website, it is permitted to tell others about it even if it may cause some damage, and this is what the Be'er Mayim Chaim 27 is referring to. But something which is derogatory such as a sin that one has transgressed, it is forbidden to publicize it even if the person himself mentioned it in front of three people or posted it on his website, because it might embarrass him, and the intention of the person publicizing it is to embarrass him. This part is written in Be'er Mayim Chaim 28.