Upvoted for effort, but if all I want to do is show off a subset of the books I've read and offer a bit of commentary on selected titles, I could do that on a web page. Why would I want to bother providing personal data to yet another web app?
I have an app I want to write. It would need crowd-sourced data as well, and there seem to be only a couple of ways to make it worth someone's while to enter that data.
One is soapboxing. Medium, for an instance. I don't have good feelings about that. The other is recommendations, but I'm not sure if that works either.
Consider the bell curve of average users, where the curve is how widely read (in this case, literally, but also figuratively) the individuals are.
If I'm out on the right end of that curve then my value to the site is high, but the value of the site is rather low to me. If diversity isn't very high, then everyone is mostly entering reviews for material I'm already familiar with. I'm bored. I get nothing except maybe a soapbox.
Perhaps sites like this should consider these prolific people to be their 'whales' and treat them the way other companies treat whales; by going out of their way to do something for them. Could be, and I'm just spitballing here, that a properly run site should be paying a few professionals to add material and make sure there's something substantive for these high value users. Hand-built recommendations, if nothing else presents itself as a solution.
I can soapbox on my own website. You might argue that the purpose of a soapbox site like Medium is to get read, but getting read doesn't matter as much to me as it once did. I've had people read my crap, and it wasn't the life-altering experience I thought it would be because nobody cared. What matters to me now is having written.
Recommendations are likewise of little worth to me; nobody knows my preferences as well as I do, and any attempt to accurately model my preferences will fail because I gain more by withholding accurate information than I do by providing it.
The truth is that I'm not really anybody's target audience.