When it comes to sending IMs instead of working, there is no match for an accountant. Accountants spend most hours in their day in front of laptops staring at excel, outlook, and instant messenger.
An amazing thing about how instant messaging works: the more explicitly company-banned instant messaging programs are, the more popular the programs become.
Accountants use MSN/Yahoo, AIM, Gtalk, and company provided programs (like Sametime and Office Communicator). These programs, as you are aware, allow one person to chat in real-time with another person or groups of people. In limited cases, accountants will constantly use e-mail back in forth in place of instant messaging. Note: that e-mail is as good of a timesink as instant messaging.
You may think instant messaging is a trivial skill, but accountants always have to take it to the next level. While dealing with others, working on memos, and reviewing workpapers, accountants also have to keep up to date on office gossip and the latest on youtube. Instant messaging allows them to stay current on the latter two.
A good accomplice of instant messaging is the privacy screen. Accountants will go to great lengths to sit with their laptop screen facing a certain direction of the room and use a privacy screen. All this careful planning allows them to surf the web and send IMs all day undetected.