The change is that the CBP agent will now look at your past public twitter statements, and (if facebook lets them) the semi-private communications of your known associates, and then use that to decide if they should detain you, deny access, etc. I imagine this will be implemented in a questionable fashion, like keyword search or "deep" learning, since the agent won't have time to actuall read stuff for each traveller.

So, you find yourself stranded in an airport because some acquaintance you haven't talked to since high school made an anti-American comment (typoed "top" as "tpp" next to an expletive) and shared it with their facebook friends. This will clearly have a chilling effect on freedom of speech (and Trump says he wants to add some border test to make sure you have "American values" shudder). Anyway, your business trip is cancelled, so you can't do your job moving forward and have no recourse.

The fact that they are looking at this stuff for foreign travelers is already way outside historical standards for acceptable government behavior. In all likelihood, they will/are running similar tests against US citizens for things like the terrorist watch list (and they want to expand the implications of being on that list, taking away more constitutional rights without trial, or any sort of due process).