It's everywhere because it's popular, it's popular because it's a well-designed web-site that makes communication and organisation with friends (as well as getting the latest info from your favourite things, such as bands announcing their tour-dates) incredibly easy.
As for store owners using it instead of it web-sites, it's only logical. Not only does interacting with the page provide free advertising (in that if I "like" the page that shows up on my profile for anyone who views it, as well as such other interactions as commenting and sharing) but it's a hell of a lot easier than setting up a web-site AND you can constantly stay in touch with your fan-base considering how often people use Facebook.
I mean believe-you-me, before the start of last year I seriously wanted nothing to do with Facebook. Then I actually checked it out, made myself a profile and the rest is happy history. Hell, I can honestly say that my social life has tripled since getting it, even with the friends I had since before Facebook (as a few of them got profiles after me) simply because something like, say, seeing a movie has been made far easier than the old rigamarole of far-ahead planning, getting down numbers over the phone and ultimately not having much freedom in planning, organising and discussing it. Now I just set up an event (or if it's just something like a movie I can just message who I want to see it with) and we're able to pin-point things down regarding the event down to satellite-view street directions up until the last minute before the event takes place.
As for why Myspace wasn't used to this effect, it was garbage, full stop.
