Twitter is the noisiest medium in history. Do you actually believe that Taylor Swift has 33,000,000 million (and counting) people eagerly waiting for her next tweet, ready to click on whatever she links to?
In fact, less than one in a thousand people who 'get' one of her tweets will click. Most of the 33 million won't even read it, making the word 'get' worthy of quotation marks.
And yet Twitter works just fine at this level. That's because it immerses the user in waves of media, a stream of ignorable content that people can dip into at will. More noise makes it better, not worse.
Email was wrecked by many marketers for many people, because email isn't structured for noise. Noise is the enemy. Instant messages, because there is no easy accessible API, isn't overwhelmed, but it too is noise-intolerant. Texts you don't want to get are a huge hassle.
The simple rule is that the easier it is to use a medium, the faster it will become noisy, and the noisier it is, the less responsive it is.
You can play at Facebook and Twitter, and make them work. But they will only work if you treat them like a cocktail party, as an opportunity to eavesdrop and layer general connection and value and insight. No, it's not an ideal direct marketing medium. It's a metropolis.