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The string itself leaks
A handle is a fingerprint made of choices. Real names, initials, birth years, city codes, school abbreviations and sports teams all appear in usernames chosen quickly and kept for a decade.
The leak is not dramatic on its own. It becomes useful to a stranger when the same string carries the same clues across several platforms at once.
The reuse pattern leaks more
One handle across twelve platforms tells an observer that all twelve accounts belong to one person, which is exactly the correlation you may not want.
Worse, the reuse is often asymmetric: a careful private account and a careless public one. The careless one becomes the thread that unravels the careful one — a linked photo, a shared bio line, a cross-posted image.
What the combination reveals
Put three public accounts together and you can often infer a rough age from handle conventions, a city from posting patterns, an employer from follower overlap, and a daily routine from timestamps.
None of that requires hacking. It requires correlation, which is exactly what a reverse username search automates.
Building identities that do not touch
The practical defence is separation, maintained deliberately: a distinct handle per identity, no reused photos, no cross-links, and no shared bio text. Three separate identities should never be able to prove they are one person.
Accept the cost. Separation means losing the convenience of one memorable name, and it means keeping track of which identity belongs where — a password manager with notes is a reasonable place to do that.