I imported roughly 60,000 users into our test account yesterday. The import said it failed for all of them. But when I search in the Dashboard, roughly 40,000 users were added. When I search for some users, I can not find them. But when I try to create them, it gives me a ‘User already exists’ error even though I can not find them or sign in with them.
On a side note, the Import extension is hardly a usable feature. It gives you no details on what succeeded, what failed, etc.