The Ottawa Ontario library search shows 1418 ePubs, 1405 PDF books, and 246 Mobi (both fiction and nonfiction). 603 ePub titles available, 832 PDF, and 231 Mobi - so it would appear only 15 people have a Mobi checked out. Some titles are duplicated between formats.
You can return an ePub but not a Mobi, which makes it seem like the ePub checkout time would be on average shorter. However it isn't readily obvious how to "return" an ebook.
The library also has around 3000 Overdrive MP3 and WMA audiobooks with 1700 available. Some titles are duplicated. These also cannot be returned.