[_asummers]

Those sites are a huge portion of overall internet traffic. While on e.g. Reddit, people may subscribe to tech subreddits, there are many people who don't and are unaware of anything to do with net neutrality. If they can convince a nonzero amount of people to contact their representatives, that's worth the hour or two of some intern's time to put up a banner. Who knows, some of those people might be in states represented by those 50 senators.