It was cool at first because I actually spent time with my "friends" on there, or at least saw them on a daily basis at my small high school. It's changed in a couple key ways over time:
1. Family joining Facebook. It used to be a funny place I could goof around with my friends, posting whatever I wanted. Now I have to filter everything I say because my grandparents can see it. I concede that that's probably not a concern for many people, but I noticed the "vibe" changed significantly when Facebook's popularity grew beyond high school/college age people.
2. Sharing. Virtually everything on my timeline came from pages that post short "viral" videos for people to share. No one was posting anything unique or interesting. It was like a heavily bastardized version of Reddit without any interesting content. Anecdotally, this content seems to appeal to older people, who don't know of ways to see more interesting content.
Privacy concerns aside, those are the main reasons that I no longer care about Facebook. I deleted mine 6 months ago and I don't miss it at all.