OT here, but I've used Picasa's facial recognition extensively on a very large library of photos (about 50,000). Anecdotally, I would say it started out very good, but, strangely, it got much worse over time.

Part or all of this effect may have been due to UI bugs or deficiencies that didn't clearly show what it was really trying to tell me (does it really think this is a new person, or separate for some other reason?), or didn't allow for subtle variations on what I was trying to tell it (such as "no, but good guess", or "this looks nothing like the person, but actually is")[1].

I can only guess why this is, but at first it seemed to be quite good at finding new faces that looked very similar to the ones it had found so far. Over time, it's as if the wide variety of confirmed positives reduced the confidence in finding any new faces at all.

[1]: It was somewhat confusing whether I should select and move a bunch of wrong faces to the right person, or just say no and let it try again. It might have also helped if I'd been able to say "yes these are all the same person, but I don't want to name them in my database".