Yes, this is part of the intent behind PatientsLikeMe's Open Research Exchange (ORE, http://openresearchexchange.com/) to help develop better patient reported outcomes by crowdsourcing input from patients on the validity and value of specific questions to be asked.
1.) Find a relevant population that's responsive and active. Note that forums, Twitter, Facebook groups, etc. can be dominated by a small, vocal, hyper-literate minority, and just one or two people with an axe to grind can skew things quite a bit. Sometimes better to field a "Behind the scenes" survey so that you get the view of introverts too! I notice you do some research in hard-to-reach populations - what are some of the inbuilt barriers and assumptions of social media they might face?
2.) Have a practise run. The first time you do this I would bet you mostly get feedback that's not directly helpful, because this type of exercise is quite new for most participants. Hopefully what you learn in the pilot can be applied towards your "live" feedback exercise with no wasted time / content.
3.) Provide multiple channels and think about barriers. "What a contentious blog post, I'll make a comment" - I have thought this many times only to be confronted with a great reason not to - registering for yet another blog site or having to log in with Facebook thereby risking putting my identity somewhere very public. You haven't said how contentious the topic is but this could have an impact on how open people are willing to be. Providing a route like an email address "[email protected]" could give you a more direct and less exposed channel for participants.
4.) Make it clear how to give feedback. How do you want them to engage in feedback? Are there words they don't understand? Do they think this project is a priority 2/10 or a 9/10? Would they take part? How would they explain it to their grandmother? Would they persuade other people to take part and if so how would they do that? Do they have any reservations about the topic? About you? About your institution? - I would use very specific questions to help elicit specific feedback. Otherwise a short open text box is likely to elicit either little feedback or non-specific feedback e.g. "Sounds interesting". Make sure there's a deadline!!
5.) Close the loop. Did you use the feedback? If so, use the same channels you advertised through to let participants know how their input changed the study.
6.) Take credit. When you come to write up the study, allude to this process in the methods such that subsequent researchers can take advantage of your learnings too. If someone has been particularly helpful in proselytizing feedback they might even merit an acknowledgement in the full paper.