Should those who study social media communities be required to inform the community of the research results?
I think that this is both an easy, yet a very complicated question! I believe that the ethical thing to do is to post to that community once your research is done (if not earlier) to let them know. If it's on twitter for example I would expect the researcher(s) to post something on that community hashtag to indicate what the research was about and what the results were. It's fairly easy (and free) to setup a Google Site or a Wikispace, or even a Weebly site to post a few pages about the research, you as researcher, and the findings. Personally I think that it would be good, for research purposes, to invite commentary from that community so that they could read what you wrote and provide you with feedback if you completely misunderstood something. This would be valuable to include in your published research.
If I quoted someone I would personally feel very weird about contacting them and saying "Hey! I did this research and I used one of your quotes anonymously". The reason I feel weird about going that extra step is that I've met many odd people (in real life and on the web) over the past twenty years who even if they said something publicly on the web, and even though I quoted them anonymously they would try to get me to remove it from my paper. I get that some people can be weird like that (heck, I have momentary "WTF?!" moments when I meet people and they tell me that they read my blogs) but I don't want to argue with someone who's said something publicly about removing something from my paper. I think it's fair game. Had they been participants in closed research where they signed on and they had the right to drop out at anytime, then that kind request is perfectly fair (even though it can mess up the research process). So, for me, not contacting the quoted people is more of a way to prevent headaches for me.
I think that internet community research has opened the door to a lot of interesting research. In many cases it's also made it easier to join communities and do internet ethnographic work. However, it's also made it much more easy for people to not do the traditional things that people do with ethnographic work, such as getting the consent of people to be studied before joining a community. Since we can lurk and go undetected by the average member, I think it's made it easier for researchers to ignore the human element and treat people online in a sterile way. I think that this is something that we in the profession need to do a better job at: humanizing online research.