There are over 900 million Twitter accounts but not everyone is actually sending tweets:

A report from Twopcharts, a website that monitors Twitter account activity, states that about 44% of the 974 million existing Twitter accounts have never sent a tweet…

Twitter said it has 241 million monthly active users the last three months of 2013. Twitter defines a monthly active user as an account that logs in at least once a month. By Twitter’s standards, a person does not have to tweet to be considered a monthly active user…

But having engaged users–those who are active participants in the online conversation–are particularly valuable to Twitter. For one thing, activity tends to make users more inclined to continue using the service.

Secondly, user tweets, retweets, favorites and other actions help Twitter generate advertising revenue. Over the last year, the company has made it easier for users to do those things and introduced user-friendly features such as pictures into the timeline…

Moreover, the report highlights Twitter’s user retention issue. It estimates 542.1 million accounts have sent at least one tweet since they’ve been created, suggesting that more than half of the accounts in existence have actively tried out the service. But just 23% of those accounts have tweeted sometime in the last 30 days.

And how many of these accounts are fake?

All together, the number of people actively using Twitter – meaning they are tweeting themselves, retweeting, interacting with others – is still limited. If you read a lot of Internet stories from journalists and bloggers, it sounds like lots of people are on Twitter doing important things. But, these users are likely a limited part of the population: more educated, have regular access to smartphones and Internet connections, younger. This doesn’t mean Twitter is worthless but it does suggest it is not exactly representative of Americans.