Not so sure it will be ineffective...
The problem for the big social media platforms may not be that this executive order may or may not be effective in and of itself. Their big problem is that it weakens their protection on one of the more contested powers on their platforms: their ability to edit or even delete self-published material, even though anything published may well not be illegal/obscene/whatever at the location of publishing.
Trump is, by far, not the only one that has run into the rather...haphazard.. and uniquely US-centric morality of "staff editing" that takes place on social media platforms. He's simply one of the top-profile ones, who doesn't have to run the infamously opague and boobytrapped hurdles to figure out what in your post has offended the Powers that Be on your social platform of choice so that it doesn't get published, or even gets you an appointment with the Banhammer.
Between the litigious nature of US society, and the rather sizeable group of people who feel Offended by censorship actions on social media platforms, the indirect effect ( and possibly actual purpose) of this executive order is the threat to open the floodgates to a deluge of lawsuits which up until now were blocked by that lovely little special protection of those social media platforms. Death by Papercut....
Trump himself may be the self-evident embodyment of the "cockwomble", but even while he may not be the brightest bulb in the bunch, his handlers are not stupid. And the threat of a ton of lawsuits that cannot be directly attributed to the Presidents' Office is a lovely little crowbar.
And so subtle that most news outlets have missed this implication outright.