The thing I find interesting about referring to Facebook as a social utility is how different that angle is than just being "social" and that's it.
Instagram (and others) may be more "cool" at this point than Facebook, but they absolutely do not provide the same level of usefulness. Private messages, events with invitations, groups, the level of organization for photos, etc all together in one place connected to everyone you care about is indeed "useful." It's just straight up inefficient to organize events and share specific information on Instagram, Twitter or like services. It's because of this I feel they will continue to thrive, as teens will still use them for the "utility" and other services for the new, trendy social interactions.
1. It's slow. I've come to expect a full JS front-end app for productivity tools nowadays, page loads feel very laggy.
2. They request your password in the email you have to send them to delete your account!!! That's the most ridiculous thing I've ever heard.