Definitely phishing. That domain structure is pretty common and used to confuse people - It looks like instagram.com with the rest of the URL behind it, but the real domain is "com-verify-1489788456268679154344343179665471433.us" as you figured out. But, to most people, they just see the "instagram.com" portion and tune out on the gibberish afterwards.
Like you said, they stand these domains up quickly and send out phishing e-mails. The goal is typically credential capture - people "log in" and the attackers record the username/password, and then the victim is redirected to the official Instagram domain.