Is it improper to pray for someone you only know online?

In online Muslim support groups, I often see Muslims talking about personal struggles and others saying they will pray for them. However, (I presume) these two people only know each other online, and this strikes me as improper: - It seems risky to pray for someone you don't really know. What you wouldn't have prayed for them if you had known more about the person or their situation? You could unwittingly be praying for something bad to happen. - It feels cheap to pray for someone known only online, perhaps known to you as UsernameX. It's as if making this prayer lowers the standard of your prayer, as if they're unsanitized and you would be willing to pray for whatever. **Question**: Is it improper to pray for someone you only know online? Since this is common, it's probably okay to do, in which case, I'd be interested in learning what's wrong with my thought process. And, since this question could be regarded as subjective, I ask that answers be backed up in some way (e.g., by quoting from Quran, Hadith, a fatwa), and not just "you should think X".