Reddit for learning!?

So we all know Reddit. Its that place that we find funny pictures and learn about random stuff or maybe browse /r/worldnews occasionally but thats all I really used it for. However recently I've found how useful Reddit is as a learning tool. Reddit has some incredibly useful boards for learning new skills. I've been playing about with some System Admin stuff as well as messing about with some networking and I've found that /r/sysadmin is amazingly handy as wekk as the /r/networking. Not only that, you've got subreddits like /r/redditdayof which encourages you to research a new topic each day and post about it. You also get that wonderful stuff called karma as well as some special badges for doing so. Just now I found an excellent reddit called /r/dailyprogrammer and it gets you to complete a challenge related to programming each day. That seems like a really useful too for us programmers although even the easy challenges seem like a bit of challenge... I think I may need to try some more of these challenges to keep me going.
The only problem I find is that you'll see some subreddits which could be useful but just arent that active. And therein lies another problem, you kinda need to discover the useful Subreddits. So far its just been finding them by just testing them or finding a link to a useful subreddit by another subreddits' sidebar. But I'm sure I'll find a better way to find a subreddit and post about it later.