Instagram went first and then I realized I hadn't logged on to Facebook in 2 years so why bother keeping it around?
I think for people who can just decide to keep the account and no longer use it, they should keep it just to stay in touch with some folks. But I realize that as long as I'd have an account, I'd keep using it.
Personally, for me the tradeoff between 2+ hours every day on Instagram for an occasional few hours with an acquaintance isn't worth it.