Everytime we "Like" something, we get updates from the thread(s) we liked, e.g: Anything majorly edited, etc.
If it was a sales thread, and you wanted to buy something later, because the amount you wanted wasn't in stock, if he/she liked the thread, they would get a PM for the edits if you understand me.
I think I get what you mean, like a post and receive private messages when the post you liked it edited. Good idea, although, like the tagging in posts feature, it should be able to be toggled in the User Cp.
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