Again, only speculation, but what I'd do.
Once they know you are coming, they'll capture your contact details (emails, phone numbers) from the airline. They will be added into the details you formally provide about your "social media" accounts. I imagine that they will allow US Federal case law to determine if an account is "social media" or not, just to keep you on your toes.
They will get the SM provide (e.g. Meta, X) to then produce a report on those accounts. including any change of contact details in the past 5 years That would produce further contact details that can be fed back into the trawl.
The people and groups you have interacted with will be pulled into the trawl too.
Once that initial tranche is available, it will be AI all the way, as they are scanned for whatever is deemed objectionable. With the algorithms being taught how to spot undesirables.
That will cover the 90% of applicants easily.
The only complication will be doing it in other languages.
Obviously they will use their powers to trawl any data held by a US company (e.g. Microsoft, Apple, Google) to cross reference the search.
Does your employer have a wiki or collaboration tool ? I would be prepared to hand those details over too.