The MongoDB Storage FAQ says to use either compact or repair to reclaim space, in the section Why are the files in my data directory larger than the data in my database. Given the title of that section, I believe that most readers will interpret this as compact being a way of reclaiming disk space.

Compact does not reclaim disk space, however. It only refragments. The compact documentation page spells this out, in the Disk Space section.

We should remove compact as a means of reclaiming disk space from the Storage FAQ.