Online debates about politics tend to be bad, but that's mostly just because of people who are more interested in propagandizing than honestly hashing out ideas. If they aren't doing that, and you aren't doing that, it can be worthwhile. IMO the easiest way to tell the difference and know to stop replying is if you get a response that clearly was written after loosely skimming your comment, and using those few words as a jumping off point to launch into a mostly unrelated rant, rather than directly addressing your central point. Or, a common one with conservatives, an argument framed as unsolicited personal advice, that's never going anywhere constructive.