True. There are some brand outliers that have a cult instagram following but relatively less web visits. E.g. Kyliecosmetics has 19 million instagram followers, 3 million web views. Forever21 has 16 million instagram followers and 22 million web views.
IG followers are definitely a good measure for the future customer pipeline judging by the % of people aged <18 on that platform. I have a 12 year old relative and she's already got 4 instagram accounts and uses sephora, mecca, and follows kyliecosmetics, kim kardashian etc.