1. Tutorial on GNOME Activity Journal, Zeitgeist and Sezen describes it as a universal user's interaction logging system.
2. I was never asked if I want to install or enable such system and/or logging during Ubuntu install/upgrade.
3. It turned out that daemon was installed and running automatically, but frontend app (gnome-activity-journal) wasn't installed.
So, this thing collected my activity without my consent and behind my back, and clearly not for the purpose of letting me use that data. This all pretty matches the definition of trojan spyware. In addition to that it also eats system resources and makes system running slow.
Few people share the concern that Zeitgeist is exactly spyware and not some productivity tool, future of Linux user interface and other marketing bullshit from Gnome3 and Canonical:
Overall situation with spyware on Ubuntu, from Richard Stallman: http://www.fsf.org/blogs/rms/ubuntu-spyware-what-to-do .