Unfortunately I would recommend against all of these (except /r/compscipapers, only because I've never checked it out).

/r/compsci is, in my experience, mostly lower level undergrads who really want to show how much they know, there's a lot of misinformation in comments that gets voted up, and a lot of missing nuance in any of the discussions.

/r/semanticweb is very inactive, almost no discussion

/r/machinelearning is pretty poor as well, better choices are http://metaoptimize.com/qa http://blog.kaggle.com/category/how-i-did-it/ and following ml topics on quora

The value there, IMO, is more the links than the discussion. It's not so much like HN, where the discussion itself is half (or more) of the value. But, for a quick, concise list of new links in those fields, I find those Reddits all very valuable.

better choices are http://metaoptimize.com/qa http://blog.kaggle.com/category/how-i-did-it/ and following ml topics on quora

Yes, those are excellent sites as well. There are also some good StackExchange sites that can help one get their fill of CS'y topics.

My favorite subreddits are http://www.reddit.com/r/reverseengineering and http://www.reddit.com/r/types .