Slightly relevant since you mentioned Quora: https://news.ycombinator.com/item?id=7796834

Scribd and Quora considered harmful

Specifically, @nelson suggests "And instead of Quora use Ask MetaFilter or StackExchange." He goes on to say:

> Quora’s business model is to trick people into sharing information for free, then put it behind a login. It’s like Experts Exchange 2.0! For instance, on Quora you can read Who owns the copyright on content contributed to Quora? Only you can’t just read the text. Depending on your history with the site and the way you got there you may see a giant popup demanding you log in obscuring the page, or the first answer clear and then the rest blurred, or if you're lucky just the page. It appears nondeterministic.

The quora experts exchange parallel is not a fair one. Expertsexchange attempted to get you to pay for membership to view the answer (it was available impossibly far down the page so google would index, but obscured as much as possible). Quora prompts you to create a free account after a couple question views, which is infuriating as a user(imho probably not going to be worth it from a business perspective either), but thats a pretty big distance from deceiving you into paying.*

*admittedly Quora's total lack of a revenue model at the moment helps them out here.

Personal data is a form of payment:

http://www.cnn.com/2014/03/31/tech/web/gmail-privacy-problem...

http://wsj.com/whattheyknow