To me, IM's represent what I like to call "One-Sided Apps". I read them quite frequently, but I rarely post to them.
For example, the author of one of my favorite webcomics posts his new ones pretty late at night. His IM, which is right next to his comic, shows his progress. So, I can know whether or not I can wait an hour and get a chuckle before bed or to wait until the next day to look.
That's probably the best application, the "Broad Audience" appeal. Since Jeph (The guy who makes the webcomic) is "talking" to thousands of people, posting an IM makes the best sense.
It also explains why I never use it. If I ever have to talk to people, it's always one at a time and I'll just call or text them. I've never had the "I need to talk to a whole bunch of people right now" scenario happen to me.
For the library, the best use for IM would be for programs. Again, that's the need to talk to a broad audience.
Skype, with the webcam and all, sounds a little too advanced for me.
I did the second option for the exercise, but tslacdl isn't currently online. So, I left a message and she should post a response soon. Then, I'll post an update.