Their lists are definitely used, which is the only way I knew about them - some years ago one major email provider was not receiving email from my DO hosted site at all, completely blackholed. I knew enough of what I was doing to know this was practically impossible, spent over a week running every kind of configuration scan and test I could think of, and finally narrowed it down to this organization in particular blacklisting all of DO IP space. Needless to say I was shocked and annoyed at such an unprofessional looking and sounding website wielding such control over the operation of my site (which, lest it needs to be said, was a completely legitimate site).