This is entirely speculation on my part, but maybe this is some insight into how many spam accounts there are on Twitter.
I am fairly worried by the security policy at Twitter.
For example, I have a friend who had his Twitter account hacked. As an experiment, he deactivated the account, but did not change the password. Whoever had the password logged into the hacked account and reactivated it. When he received the email of the reactivation, there was no "If you did not initiate this, click here" option.
There really ought to be some general rule of account classification in a broad-based public service. My very rough rule of thumb:
10% of accounts are active (daily/weeekly participation)
1% of accounts are "whales" (provide high level to the service).
~15-50% of accounts are some-time users.
~25-50% of accounts are one-time users (registered but never used)
If your service is sufficiently old, call it 5-10 years ...
~25-50% of accounts are expired / no longer reachable (usually the contact email/phone is no longer valid).
Active spammers don't have to be a high level of the service to be disruptive, but can be anywhere from 1-25%, mostly depending on how effective you are at rooting them out.
Very, very rough, and no, I don't have a particularly good basis to back these up other than the first 2-3 values.
I am fairly worried by the security policy at Twitter.
For example, I have a friend who had his Twitter account hacked. As an experiment, he deactivated the account, but did not change the password. Whoever had the password logged into the hacked account and reactivated it. When he received the email of the reactivation, there was no "If you did not initiate this, click here" option.
edit: formatting