Necessary background: Lashon Harah (slander) is prohibited, unless it is for a to'eles (constructive purpose).
Can I say Lashon Harah that would be available to everyone, would be beneficial for some people, and I have no way of finding the people for whom it would be beneficial?
For example, let's say I am renting an apartment and my landlord is a miserable wretch. I would like to warn any future tenants about his wretchedness. However, he won't be sending any future potential tenants to me as a reference. Can I post to an anonymous internet site how terrible my landlord was, in the hopes that future potential tenants may google the address and find my review? The information would also be available to people who weren't looking into this apartment.
Would it make a difference if the site I posted to was a primarily Jewish visited site (such as Yeshiva World News or something) or if the landlord was Jewish or not?