How are these users deleted? Is there any threshold after which they are automatically removed?
Manually once they notify us.
There are some pitfalls to the process. Most of them come down to communication issues. For example, what some people do is email us from an unrelated and/or mistyped email without linking to their account, so we have no idea who they are and can't respond to them. Or they have several profiles they'd like to remove, but they don't mention that when contacting us. Those accounts will probably languish forever.
Some emails used to get trapped in the spam folder never to be seen again. (We get an impressive amount of spam every day.) Using the "contact us" link at the bottom of every page should theoretically fix this specific issue, but some folks still email us directly. Spam-trapping is rare, but I figured I'd mention it for the sake of completeness.
Our typical response time is a couple of days, so if someone doesn't hear from us in a few days, they can ping us again to see what's what.
As an aside, we do automatically delete some abandoned accounts. The specific rules for that are described in this blog post.
a) Forgot to flag something or email them to complete the request, or