Maybe this [0] and this [1] fits the bill, both of them have very few users.
I still don't see how mobile will make it work, you will still have no idea if there is something going into the button and as I said if you go there to read something you will never ever find another comment because of the massiveness of the web. Even if you write a review, who is it for? The chance of someone reading this review, on this app, on the specific page for a product they want is so small it is not even funny. And writing a review usually means you do it for either the "stats" of writing reviews which you now don't get, or for others to read, which they just won't.
Don't get me wrong, I'd love for something like this to succeed and thrive. I just don't see it happening right now.
[0] https://chrome.google.com/webstore/detail/dropchat/eaejhpdja...
[1] https://chrome.google.com/webstore/detail/retort/ianfcbdiagd...
I had an idea that you could have a feed where peoples comments are showing up. So when you'd press the extension or open the app you'd see "oh people are using this app and this is where they are commenting".
Then Birdy itself becomes the "network" and each comment made is more likely to be seen by other Birdy members.