The most active users are publicity-seeking celebrities, media outlets, companies and authors (guilty as charged!). These are people or organizations who have a vested interest in frequent tweets. And they dominate Twitter.
Most people don't have a compelling reason to spend the time broadcasting to strangers, but get value from the publicity seekers.
I think the media hype around Twitter was getting ready to naturally subside, when talk show star Oprah Winfrey made a big stink about Twitter on national TV. This happened in mid-April, which was enough to push April numbers into the stratosphere, matching previous months' growth.
Nearly all the people responsible for the tidal wave of sign-ups and traffic to www.twitter.com subsided by the end of April, leaving poor Twitter looking like an abandoned warehouse in May. In fact, I think that if you remove the Oprah traffic, it would have been a much subtler transition from March to May.
Also: I think Oprah was Twitter's jump-the-shark moment for the media. Once Oprah tells her TV cult audience that something is the new hotness, it is at that moment officially old and busted.
Still, the rapid growth is slowing. Why? I think one reason is that several types of users have become turned off by Twitter. First is the analytical social networking professionals that dominated a lot of the conversation in the early days. Twitter has become a stadium, rather than a brainstorming room, and so they've moved on to services where they can have a meaningful conversation -- FriendFeed, for example.
Trend chasers have also moved on, now that Twitter isn't trendy.
It's also worth pointing out that many of the most active users don't visit twitter.com anymore. Many just log in using a cell phone app or third-party application. I'm one of them.
I used to use Twitter via Twitter.com exclusively. Now I use one of two other third-party services for posting, and another desktop app for viewing Tweets. Half the time, I'm using my iPhone. I hardly ever visit twitter.com, so all my activity isn't measured by some of the studies.
In a nutshell, Twitter is normalizing. It's no longer a new frontier, an elite club or a culture-transforming medium. It's just a service for sending messages.
As Harvard correctly figured out, Twitter is mostly a one-to-many medium. Young adults, who grew up with online communication, have no desire to talk to strangers. They love communicating with friends and acquaintances, which is why social networks like Facebook are overwhelmingly preferred.
Twitter is appealing to people with something to sell, or people who want to network professionally. It's also a great way to follow a hobby or intellectual interest.
In other words, it's for older people, mainly.
So is Twitter dead? Far from it. But the Twitter hype bubble has surely burst (thanks, Oprah!). Now those of us who actually get value from it can enjoy it with less of the hype, expectation and noise than we've been seeing in the past few months.