When the exciting new thing called social media first came along, it promised to do what years of reality television, forests' worth of glossy magazines and countless fish-suppers' worth of paparazzi shots failed quite to manage: it would allow celebrities to show us The Real Them.

If you ever wondered what really went on in the heads of the people you are used to goggling at on telly, you needed wonder no longer: now, thanks to the wonder of Twitter, we would be able to SEE DIRECTLY INTO THEIR BRAINS.

It seems to work; at least for celebrities who write their own tweets. You discover that Simon Pegg is funny and nice, Graham Linehan intelligent and politically conscious, William Gibson geeky and sociable, Amy Winehouse a bit erratic, and 50 Cent . . . well, you discover that 50 Cent is an absolutely epic plonker.

He is rapidly becoming a reason in himself to go on Twitter. It's grammatically haphazard – as he says: "Any sucker can press spell check" – but you tend to get the gist. "A yal be on twitter meeting each other. Then yal be fucking this shit is crazy. I wanta find me a bad bitch on twitter. Lol" is pretty much average.

In the last week or so alone he has told us about having "shaved the poodle", encouraged female followers to tweet him pictures of themselves in their bras and pants, speculated ungallantly on the private parts of other artistes (Erykah Badu's, he says cryptically, "make a nigga colour blind"), and announced the formation of a three million-strong cult led by, er, him, with sketchy proposals for a eugenic breeding programme.

A very useful supplementary feed – @English50Cent – interprets his sayings for those less with it. For instance, when Fiddy found himself having an online scrap with some pre-teen Justin Bieber fans, he tweeted: "I'm a take my belt off and beat one of you little motherfuckers were your mama and daddy at anyway bad ass kids." @English50Cent translated: "I am going to remove my trousers and attack some children."

This is all glorious in a horrifying sort of way. But is this 50 Cent making his own myth or undermining it? From time to time you can tell – or imagine you can – that a member of Fiddy's entourage has risked life and limb to physically wrest the iPhone from the boss's grip and started tweeting on his behalf.

He spells better and becomes more philosophical. "To hate me is to hate success," he says. He adds that university degrees are a better test of short-term memory than underlying intelligence. And he warns: "I do believe that a wise man who plays the part of a fool will learn faster."

Some years ago, Giles Foden was banished to Pseud's Corner when he used these pages to compare Eminem to Robert Browning. But he was on to something, really. He argued, quite reasonably, that the songs should be understood as dramatic monologues: Slim Shady's antisocial tendencies are no more reflective of Marshall Mathers's true feelings than the wife-murdering narrator of My Last Duchess reflected those of Browning, in real life uxorious to a fault.

Pop music has always been about projecting a persona as much as about putting over a song, and this goes double for rap. My theory is that you need to look further back than the Victorians, though: gangsta rap is basically Anglo-Saxon heroic poetry, only with phat beatz instead of fat Geats.

In gangster rap, as in Anglo-Saxon verse, you've got a poem or song – semi-improvisational, sound-patterned with rhyme or alliteration – designed to inflate an already preposterous reputation. The archetypal hero is a boastful fellow, distinguished by three things: being able to drink more mead (or smoke more weed) than everyone else, amass more gold (bling) than everyone else, and kill more enemies in fights than everyone else.

The Anglo-Saxon poet, arguably, was a little more respectful of women and less likely to insert the disclaimer "no homo" into an account of male companionship than his modern-day heirs, but the point basically holds.

What effect does social media have on the process? Deflationary, I think. The Beowulf poet wrote in the third person, but 50 Cent does so in the first. Beowulf had a scop (poet) to mediate his great deeds to posterity; Twitter goes out direct.

Does a Twitter feed ironise the image created by artist and record company, then? Is Fiddy the philosopher-king, Fiddy the Bieber-basher or Fiddy the poodle-shaver the real one? Far from allowing us to see directly into his brain, we may be no closer to knowing the real 50 Cent after all. But we feel we are, and that's somehow diminishing.

"Hwaet!" is what Beowulf would have tweeted to his adversary. "@grendelsmom I'm a beat you lol." Lol perhaps, but on balance the original has more oomph.