They can through algorithms, which is something that is painfully lacking on most social media sites (including HN). Give users the ability to dislike something strongly enough that anyone who voted it up is excluded from all vote counts/rankings/recommendations as provided to that user for some period of time. While this leads to a filtered world, that is exactly what users expect from sites like these. Reddit is es…
Second, and more interestingly: people have different facets that they show in different subreddits. As an example here on HN people prefer tech-oriented stuff. But not everyone here is that into tech. They just sort or organize the content, so they come to HN for in-depth stuff, and (say) Engadget for rumors, and sbnation for sports etc. So someone who upvotes a stupid FB pic in /r/funny might vote the same photo down in /r/politics. If the person is "sorting" content that's appropriate in different situations, it's not useful to discount their opinion entirely.