In order for people to maintain more than a few friends, it's nessasary for them to know what those friends are doing. If you have More than 100 friends/acquiantances, then talking to them say once a week or once a month is not time efficient.
Personal blogs and broadcast emails helped solve this problem, but the time taken to compose posts means that they are often infrequent and irregular.
Twitter (and Facebook) solved this by lowering the barrier to publishing by allowing it work from mobile and be short with no text formatting. Facebook was focused on a closed group of friends which left a gap for Twitter to target people who want publish to a wide number of people which includes very many people who have their own companies or otherwise see a lot of benefit from being well known.
I think Twitter executed well with the following:
- 140 character limit which forces people to be direct as non-professional writers tend to go on too long.
- Raising money early and therefore having a big team to build the product.
- API, this allowed massive innovation to happen outside twitter and to fill in the long tail of applications for this service in a way that no other site could match and they would have not been able to do internally.
- Keeping the product simple to use
- Publicity