That said, limiting the free account to only 3 feeds makes it difficult to judge the site's usefulness. I guess that I would probably want to try the site out with at least 50 feeds to see if I wanted to pay for the ability to track even more feeds.
Consider that it may be more valuable to give out fewer invitations but raise with numbers of free feeds per user.
Out of curiosity how many feeds do others subscribe to?
This post (http://news.ycombinator.com/item?id=248623) suggest you, soundsop, might be on the high end for HN readers--that's a _good thing_, you're our target market. Are there a handful of feeds that have the lowest signal-to-noise ratio? Maybe Feedscrub could help you with those in particular?
How well does this service work in practice? I imagine telling spam and non-spam emails apart is far easier then "predicting" which blog post I will like.
You're right, it's a lot easier to filter out spam emails with the word "V1AGRA" than it is to filter out news posts. You'll want to take some time to train the filter properly before you unsubscribe from your junk feed.
Do you do any calculus on the relative values of false positives and false negatives? It seems that a person with a large number of feeds would put a greater importance on limiting the number of false positives (non-interesting messages classified as interesting by your program), whereas a person with a small number of feeds would be more worried about false negatives (interesting stories classified as uninteresting).
Also, see this comment in this thread: http://news.ycombinator.com/item?id=435837 . For something like this, I doubt a machine learning algorithm will work as well as social algorithms. Web search is a very similar problem (find the most interesting link given input from the user) and social algorithms won out. Of course, the kind and quality of input that you are getting from the user is different. However, your problem is much less well-defined (finding interesting things in general instead of interesting things related to a particular topic).
