> You haven't gotten us anywhere near the point where they can compel production of social media accounts during a stop
Well... but they're doing it. The memos tell them to demand social media accounts, and it warns officers that failing to collect this information might come up in reviews and have negative impacts on their career. So we're at that point, regardless of what they are or aren't legally able to do, and regardless of what a judge would rule during trial.
Of course, it's good for people to know their rights; it's good to educate people about what the law says. And sure, the average low-wage, stressed out, busy commuter can roll the dice about what will happen if they say no. Maybe they won't get arrested, maybe they'll just get a harsher ticket written up. Maybe the cop will get visibly angry and demand that they unlock their phone and then hold out their hand, and that person can then try to overcome an entire lifetime of social conditioning to avoid placing their phone into that hand. And maybe the cop will shrug it off and walk away. It's a fun gamble.
But the base action of a cop saying to someone, "you need to give me this" is still going to happen to people regardless of whether or not you say it's supposed to happen. It is good to educate people about their rights, but rights are not a panacea against police abuse.
The awful thing is that it's entirely rational for people who are scared, who are busy, who are stressed, who are poor, and who don't want a confrontation to decide that they're not going to roll the dice and that when a cop tells them to unlock their phone or write down their Facebook username, that it's better to comply. For some people, that might be the correct choice, because antagonizing a cop is too risky for them. In a situation with a large power imbalance, "demanding" or "compelling" information doesn't always need to be backed up by a specific law. It's enough for a cop to tell you that you have to do something, with the implicit suggestion that they could arrest you or injure you if they wanted to, even if they aren't likely to do it.
So saying that cops aren't allowed to do something (while completely legally correct) still doesn't mean much unless you also have a working enforcement strategy that's going to prevent them from doing it. Eventually getting your case thrown out in court over an improper stop is still going to be a traumatic experience for most people, it's still something that someone scraping by on minimum wage can't realistically afford to risk.
You point out (accurately) that arresting someone over this would be uncommon. But this is still a situation where one party might be risking something life changing, and where the police officer is realistically risking very little. Nor does an officer need to arrest everyone who refuses the request, they just need to have a credible threat that they could arrest you. A power imbalance is enough on its own to allow an officer to demand something that most people see as a small concession -- they don't need laws to back that up.