Fellow graduate student discusses problems but gives no credit afterwards
What you describe doesn't sound particularly unreasonable or problematic to me.  Researchers discuss things with each other all the time, and of course the results of such discussions often show up in published research.  That's what you ought to expect when you have a technical discussion with someone.  In fact it's the best case - your contribution was useful and advanced science.

You ought to have been acknowledged in the paper, but the standard thing would be to have a brief section at the end, with something like "The author would like to thank House Ninja for helpful discussions."  If this was omitted, I would guess it's most likely due to C's unfamiliarity with publishing practice - he is a grad student after all.  You could have a brief word with him about this, or ask his supervisor to mention it to him.  But it isn't reasonable to expect that the paper will contain a detailed catalog of which specific idea was due to whom.  People don't keep track of things like that.  And you shouldn't expect this sort of credit to advance your career in any tangible way - it's a mere courtesy.  Such an acknowledgement wouldn't make your interaction any more or less "exploitative".

If you were expecting to become a co-author on the paper, then you should have said so explicitly at the time of the conversation: "This is really interesting to me.  Would you like to work on a paper together?"  Of course, in that case, you should expect to contribute a dramatically greater amount of work than just sharing ideas.

So I think what you gained from the discussion was about the most that you could expect - you made a contribution that helped to advance the state of the art.  You can decide if that feels like a sufficient reward for the time you spent.  If not, then maybe in future you want to spend less of your time having such discussions.  

But I wouldn't see this as exploitation.  Researchers talk to each other to get ideas.  That's the point of research.  Maybe someday C will be able to share some interesting ideas with you.


  While the other students would talk about everyday life topics, current events, family, etc., C would only ask us questions about his own research.


So, C happens to be more interested in his work than in mundane topics.  I don't think that's a problem.  You'll meet a lot more such people if you stay in science.  Maybe he wouldn't be an interesting guest at a party, but it doesn't mean he's exploiting you.