mibextid shows up on links that were never part of any ad campaign, which is exactly what confuses people about it. Someone shares a post link from Instagram or Messenger, a friend pastes it somewhere else, and there it is: a strange mibextid parameter attached to a link nobody paid to promote.
What mibextid actually marks
Meta's own apps append it when a link passes through their native share sheet, actions like "Share to...", "Copy link" or "Send in Messenger" inside Facebook, Instagram or Messenger, rather than anything set by an advertiser in Ads Manager. The value records that a share happened through one of those in-app mechanisms, and which one, feeding Meta's internal analytics of how content moves between its own apps. Meta has not published a full spec for the parameter. Treat any claim about its exact internal structure, including this one, with some caution.
mibextid versus fbclid
These two get confused constantly, but they mark different events. fbclid is appended when someone taps an outbound link inside Facebook or Instagram, and exists to match that click to a conversion for advertiser attribution. mibextid is appended when someone shares or copies a link from inside a Meta app, ad or no ad involved. A link can carry fbclid, mibextid, both, or neither, depending on whether it came from an outbound tap inside a Meta app, an in-app share, or a plain copy and paste from a browser address bar.
What to do with it
Strip it and the destination page still loads. Nothing about the visitor's experience changes. On the marketing side, mibextid rarely shows up as a usable dimension in third-party analytics tools, since it is a Meta-internal signal rather than a standard UTM field most platforms know how to parse. If you are trying to trace where a share actually originated, igshid is the closer Instagram-side equivalent. Neither parameter needs stripping before you pass a link along.