Kim et al. :
Yahoo is not perfect but I would prefer it to most alternatives.
Twitter I tried briefly to get twits from a cousin and found the length limits to be very frustrating, I deleted Twitter from my laptop and from my iPhone and will not use it under any circumstances.
Facebook is an excellent way to see pictures of the children and grandchildren of my cousins but I do not think it is a good way to convey serious information immediately, even if it is a closed group. Additional problem is that Facebook also funnels ads to you based on content. I was looking at info on Facebook page of an Islamist group and for the next several weeks was getting ads for some Islamic dating site. Rather fascinating as it involved pictures of women with women wearing veils with only the eyes visible. Some did have pretty eyes however.
I have never heard of Reddit but guess it is something like Twitter.
I always check Yahoo posts from the group. I could not tell you the last time I checked the groups FaceBook page.
I am a retired full time EMA director (for 27 years) and am interested in the group but will likely drop out if it leaves Yahoo.
Best of luck whatever you do. It is nice to see anything that helps increase situational awareness.