I wonder what the common factor between these people is (besides their skin-tone I mean).

Because surely there must be some kind of census or survey or company that had collected phone numbers and people's race information at some point. And it either got leaked or was intentionally created to harvest the phone numbers of black people?

My assumption is that some data broker had a list of age / ethnicity / phone numbers for targeted advertising.

Facebook signups -> "trusted 3rd party for serving you relevant ads" -> Jimbo's Totally Legit Web Marketing Service -> mass racist texts

It is entirely possible it was coordinated directly with bad actors / Elons at the social media companies.

Facial recognition on social media is a thing.

Race recognition from social media? Definitely possible. But linking that to a mobile number is the interesting part.

People have bad cyber hygiene in general but most people know not to post their mobile on their social media publicly.

The question that comes to mind is what percentage of black Facebook users have their phone number on their profile (only visible to their friends) and have also friended a bot? (or joined a group that compromises their security?) A fraction of a percent is still enough to do some harm.

All you'd need to do is join dark web data to social network data. Phone numbers are common parts of those data sets.