DM messages would feel rude, patronizing, and ad-personam ...
This can also apply to offering opinions publicly without invitation.
Over at another forum I frequent, they've got this golden rule plastered on two places in their guidelines:
Critiques should only be offered if requested by the original poster.
It's in place to prevent a negativity tornado from brewing. Sadly, some humans can't resist twisting feedback into a weapon against those they do not care for.
A positive step is that anyone can start a thread specifically for critiques.
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Regarding the "Like" button, I happen to like it.
When I leave a Like, I tell the photographer I noticed their work and appreciate it.
If I leave a "Love," it is the equivalent of placing a silver or gold ribbon on their photo like received in art school.
If I pass the photo by without leaving notice, that also says something.
People will have differing opinions on all of this, and this is mine.
The one thing that works to help anyone pursuing an art is encouragement.
If my Likes and Loves help give a fellow photographer some energy to continue onward, I have succeeded.