This is going to depend a good deal on there exact nature of the clips being reused. it will also depend, at least somewhat, on what country or countries the various parties are in.
General Principle
The general principle is that a peerson's post on social media is protected by copyright, just as all original works are. It is a copyright infringement to copy and redistribute the, unless permission has been granted by the copyright holder (normally the author in the case of a social media post, as those are rarely sold) or an exception to copyright applies.
Permission
There are various ways in which permission might be granted.
The re-user might directly email (or otherwise reach out to) the original author and ask, and be granted permission.
The original author might have released some or all of his or her posts under a permissive license.
Some social media might require that posts be released under a permissive license, or otherwise automatically grant such permissions. Remember that Wikipedia, for example, mandates that all posts and contributions be released under a CC-BY-SA license, although posters have the option to add an additional alternate license, or more than one.
Exceptions
The most obvious exception is Fair use, if the parties are (or at least the re-user is) in the US. People sometimes speak as if fair use is a world-wide legal rule. it is a very specifically US legal concept. Some other countries have a somewhat similar concept, known in English as "Fair Dealing". It is, i understand, rather moore limited than fair use. This law.se question and its answer discusses fair use in some detail.
Most of the posts being reused here have no market, and so their re-use cannot harm the market. That would weigh for fair use. If the re-user uses the whole post, or a substantial part of it, that would weigh against fair use. If the re-user inserts significant comment on each re-used post, that would be analytical if not transformational, and would weigh for fair use. If the re-use is monetized, that would weigh against fair use somewhat.
In short, we cn't tell in general if fair use would apply to any particular post being re-used. it might or it might not. Specifics will control, and ultimately it is a judgement call to eb made by a court, if the matter comes to that.
Enforcement
Unless the copyright holder for a given post files an infringement suit, or files a take-down notice, or complains to the host site, there will be no enforcement and no consequences. If a complaint or a take-down is filed, the host could remove the content, cancel the re-user's account, or both. If the user has monetized the re-use, ad revenue can be redirected to the copyright owner.
If the copyright holder files an infringement suit, and wins, there will be an award of damages, which might be quite small, or rather large. But with no market for the original, and probably little or no income from the re-use, low damages are more likely than high. And the original copyright holder would have filing fees, legal expenses, and possibly other expenses in advance.
There are firms with which one can register one's copyrighted work, who search for it being improperly used and take action, in return for a small up-front fee and a share of any damages recovered. But posters of general opinion on social media seem unlikely to use such services.
Conclusion
Such a “Social Media Reacts”-video is somewhat risky, from a copyright perspective, but the likely negative outcomes are not large. Asking permission whenever possible, and trying to stay within the fair-use principles (if they apply) will reduce those risks.
