In the case of reddit, it's userbase is very difficult to monetize -- they've said as much themselves. I think the less you can monetize something the more likely you'll have a lot of users; once you start doing things to monetize your userbase they get annoyed and simply move on.
I think any one of these sites is bad investment because those 300 million users can come and go in an instant. Digg could simply be over now and just takes a few people a few months to throw out the next big thing.
I agree with you, somewhat, but I will play devil's advocate and point out that for the first half of the Internet age, search was considered "very difficult to monetize". So much so, that all the major players started building portals and other BS to make their search more monetizable. This shift left a huge gaping hole for Google to fill, because there were almost no pure search engines left! Digg has gone down a similar path...building up and out to try to send tentacles out into more monetizable spaces, rather than having a focus on their value to their users. reddit remains extremely valuable to its users.
I don't know how they'll monetize it, but I can't help but think that whenever someone does figure out how to monetize a community of that size, it will be pretty serious cash.
In the case of reddit, it's userbase is very difficult to monetize -- they've said as much themselves. I think the less you can monetize something the more likely you'll have a lot of users; once you start doing things to monetize your userbase they get annoyed and simply move on.
I think any one of these sites is bad investment because those 300 million users can come and go in an instant. Digg could simply be over now and just takes a few people a few months to throw out the next big thing.