The problem is that we've equated some platforms with a certain kind of media. Youtube is video, Twitter is news, Facebook is.. reality tv, I dunno. It's pretty debatable whether those platforms are the right place for the coverage of war crimes. I'm not going to start an argument about the media not doing their job right. However, this discussion would be moot if there was a news platform on par with the above, and…
If your evidence for war crimes is Twitter, there's a problem. And the problem is not Twitter.
Twitter and Facebook posts should not be evidence of anything. That stuff is so easily faked that courts would be entirely right to laugh such "evidence" out of the courtroom.
Evidence should be gathered by appropriate authorities and kept in accordance with international standards on evidence storage.
Hint: That's not "look at this tweet I got!"