I read several blogs and Seth Godin (author of The Purple Cow) had this to say the other day:
There's a lot of it.
One reason: it's easy to become a poet. Easy to announce you're a poet, easy to get a pencil and a paper, easy to publish your work online.
There's a lot of bad tweeting, bad marketing, bad facebooking, bad emailing (I would also add: Bad Photography!) and bad music now as well. No barrier certainly leads to a lack of selectivity.
Surprisingly, though, amid the bad art, we actually find more good art. A barrier to entry isn't the only thing that improves quality. Sometimes it's sufficient to let artists do their work without a gatekeeper.