Exactly a month ago today, an unarmed black teenager named Michael Brown was fatally shot by a white police officer in the town of Ferguson, Missouri. There were a number of eyewitnesses, but the circumstances of the killing are disputed and are currently the subject of FBI and grand jury investigations. What is not in dispute is that the killing sparked waves of protest, rioting, heavy-handed policing, the imposition of curfews, the calling out of the National Guard, and, at times, images that made Ferguson look more like a suburb of Baghdad.

To many of us, this brought back memories of the savage beating administered to Rodney King by officers of the Los Angeles police department in 1991. This incident, which in normal circumstances would probably have been just a routine event for the LAPD, sparked the most serious urban rioting seen in the US since the Watts riots of 1965. What triggered it was the fact that the assault on King had been video-recorded by a witness from the balcony of his apartment. The resulting video is of poor quality but nevertheless makes sickening viewing; when broadcast on US network TV, it led to an explosion of rage by black citizens.

So an early example of "citizen media" played an important role in the King story. Now spool forward 23 years to 9 August this year. Ferguson is a predominantly black town, but its police force is predominantly white. Shortly after the killing, bystanders were recording eyewitness interviews and protests on smartphones and linking to the resulting footage from their Twitter accounts. News of the killing spread like wildfire across the US, leading to days of street confrontations between protesters and police and the imposition of something very like martial law. The US attorney general eventually turned up and the FBI opened a civil rights investigation. For days, if you were a Twitter user, Ferguson dominated your tweetstream, to the point where one of my acquaintances, returning from a holiday off the grid, initially inferred from the trending hashtag "#ferguson" that Sir Alex had died.

There's no doubt that Twitter played a key role in elevating a local killing into national and international news. (Even Putin's staff had some fun with it, offering to send human rights observers.) More than 3.6m Ferguson-related tweets were sent between 9 August, the day Brown was killed, and 17 August.

Three cheers for social media, then?

Not quite. Listen to what John McDermott, a thoughtful observer of new media, has to say. "If you have checked your Facebook feed over the past week and a half," he writes, "you have almost undoubtedly seen a friend participating in the ice bucket challenge, the feelgood viral sensation aimed at raising awareness of and dollars for ALS research. If, however, you spent more of your time on Twitter, you were more likely to have received harrowing updates from Ferguson, Missouri, where police officers (and now the National Guard) are trying to enforce order on a town demanding justice for Michael Brown, the 18-year-old man gunned down by a police officer on Aug 9."

One of the most important functions of social media such as Facebook and Twitter is the way it "refers" (ie point) friends or followers to articles published elsewhere. McDermott cites figures from SimpleReach, a social media analytics company that works with hundreds of publishers, including Forbes, Gawker, the New York Times and Vice, which show that "stories about Ferguson and/or Michael Brown published since Aug 7 have generated fewer Facebook referrals on average (256) than stories about the ice bucket challenge (2,106)".

This is interesting in a number of ways. First, it undermines the general assumption of commentators, especially since the dawning of the Arab spring, that all "social media" can be lumped into one category. The reality is that Twitter and Facebook are radically different networks.

More importantly, the discrepancies between the way the two services handled the Brown killing illustrate the ways in which social networks can highlight or downplay news stories or events. The idea that these technologies provide tabulae rasae on which events inscribe themselves is bunkum.

And there's no smoking gun here, just a smoking algorithm. What you see on Twitter is determined by who you follow. In contrast, what you see in your Facebook newsfeed is "curated" by the company's algorithms, which try to guess what will interest you (and induce you to buy something, perhaps). Having a frank discussion about the racism that disfigures America might not fit that bill. Which is why Facebook is for ice-bucket memes and Twitter is for what's actually going on.