Sensible measures to ethically use freely available, but personal web-based comments in research?
Talk to your Institutional Review Board (IRB). You will not be allowed to use any of the information you are collecting unless your research plan has been approved by your IRB, and they will likely require you to deidentify or anonymize data. What exactly is sensible or not, we cannot advise you here -- it's not a question of common sense but of what you can get approved by your IRB, and approval by the IRB is the single important factor in what you can and cannot do.