I've collected metrics from a few 1000 openings, analysis of recruiter data. MOST (~70%) candidates "fall off" - that is the relationship concludes with no follow-up and also no notification of "declined". Ghosting also happens from the applicants too (but much less common) ~10%
This has happened to me in the past. It is especially annoying when it is a technical assignment given and you take the time to do it then you don't hear back from them. I should have know better because the recruiter was not very responsive from the get go(this was not some tiny start-up either).
I am now of the opinion that recruiters should pay people for their time now. I will no longer be doing such recruitment exercises.