Like others said, those apps are pretty much games. The problem with downloading them is that pretty soon you will receive notifications every time someone plays with them. At least in some sorts of apps. I generally ignore them, not because I am trying to be rude, but because I have no time for it.

Facebook is different from LJ in that it's not a blog. Basically, it's a good way of staying in touch with people quickly, and some use it to post pictures. :-)