Looks awesome. I hope you succeed but the average person is going to be weary of accidentally auto-sharing that "article" they were reading on redtube.com
Thanks! Automatic sharing is opt-in instead of opt-out, which means you would have to manually add a site like redtube. On top of that, we've prevented our software from ever touching certain sites, including search engines, webmail accounts, banking or financial institution pages, and adult websites.
We don't ask for permission to post to your Facebook feed, so FB can't change anything there. Facebook definitely tracks the last time you used an app -- they expose it to you via the Application Settings tab (http://www.facebook.com/settings?tab=applications). I don't know what else they're tracking behind the scenes though. That's why we offer a non-Facebook login option.
How well does this take in information from other similar things in Facebook? The Guardian is the paper I see most in my feed, does Lookmark take note of those?
We're not scraping much data from other sites yet -- we wanted to focus on the core experience of "read what your friends are reading." We plan on taking advantage of more data sources in the future, though I don't think any Facebook Social Reader apps have open APIs.
I think having public accounts or "topics" you can follow would be a great step to fill in the gap for folks without many (interesting enough) friends on the system
Happy to answer questions/get feedback here or via email -- team@lookmark.com. Thanks for checking it out.