Uh, they kinda did copy Facebook shamelessly on the core features and basic UI, with the exception of Circles. I log in, I see a feed, I have a way to add “friends”, I can share text/photos/videos/links and comment. It basically looked and felt like logging in to Facebook with no friends on there, less features, and a few interesting UI differences.

If they had done something different, like say restricted you to X number of friends, for any small X, or focused on groups, or done something anonymous like Secret - now that at least would have been different. It may have still failed but at least it would have been trying to scratch a different itch on the social spectrum.