- Not all systems make user data (or all user data) visible. UUIDs make guessing or traversing the address space harder.
- Certain types of attacks are easier given known, guessable, predictable, or sequential identifiers. UUIDs mitigate these attacks.
- Certain types of system confidentiality are generally easier to maintain with UUIDs. (Unless, of course, you then hand over the full listing, as Google did with G+.)
- Where merging systems, UUIDs may (or may not) help avoid namespace collisions. (Merging disparate systems with independent UID conventions is a notoriously fraught problem.)
On sites helping search engines: so far as I've been able to suss out, Facebook actually leans strongly the other way. I don't seem to get much insight to Facebook by trying to do site:facebook.com limited searches (through DDG, which is to say, Bing, or Google). FB's view seems to be that if you want to see or search content on the site, you create an account and go there through the account to find content. I don't often find myself interested in searching FB myself, but for as large a site as it is, it seems to turn up in results remarkably less often than other notably closed/annoying domains, say, Scribd or Pinterest.
Even as of 2015, FB's representation among various traditional and social-media options for a set of arbitrary interest queries was relatively modest (though higher than I'd recalled before going back to look at my results) and I believe (based on impressions rather than focused research) it's become more search-engine opaque since:
https://old.reddit.com/r/dredmorbius/comments/3hp41w/trackin...