RY – while it is true that most things are permissible when said under certain conditions, I don’t think your example is the best example since it is not negative in any case.
I would think that a better example would be if someone asked you if the clothing in a certain store is good quality or if they asked you which of the ice cream places in Lakewood has better ice cream, etc, and since it is l’toeles you may be allowed to respond (although it may not be so simple, so one should ask first).
The problem in this case is that since it’s a public setting, even if it’s l’toeles to tell Rebshidduch something, it would still be assur to post it online.