No doubt it indeed has replaced email and IM for some people, but I would think it is just because "well, everyone else is on facebook so I'll just use that too"...
It's better than e-mail because I don't have to sift through spam in my private message box. I don't even have to sift through messages from companies that I technically have a business relationship with. I only get actual personal contacts.
It's better than various forms of IM software because (1) most of my relatives actually use it and (2) it's in the browser rather than a standalone client.
Of course, YMMV.
Mind, I don't use FB, or trust it, for this.
For my tech-savvy parents and non-tech-savvy grandparents, it provides an easy way to share/view photos. Using email/IM means having to come up with some sort of filing system for photos, whereas Facebook just provides it, along with captions which can be useful for travel albums.
It's a broadcast system with less urgency than email. If there's some photos or a story from daily life you want to share, emails like that will get lost in an overload of tasks and requests. IMs are easy to ignore and hard to dig up. Facebook provides a way to find out that hey, something interesting happened, and when I get around to it I can browse what's up with people I care about.