Oddly enough, that's exactly what happened to me, although mostly because I found LJ more convenient. Instead of having to go to several different websites to check on all of my fandoms, I can just go to my flist and see what's new in my fandom communities. Not only that, but I can also see what's up with my non-fandom friends. I've always been more of a lurker, anyway, and rarely post to discussion threads.