It's a twofold problem, I believe. People are lonely because of fear of rejection and also actively avoid new people out of caution and high standards.
So two people who are otherwise lonely will make no effort to connect.
I think social networks have done a tremendous amount of damage to our collective psyche. Because on the web, you can single-click permanently block someone and never see them again. If you are admin of a group this person is in, you can also ban this person and prevent them from interacting with members of the group (in the group, that is, you cannot control private messages, but by banning someone from a community you are effectively isolating them), and I think we haven't considered how much power we are giving to random Reddit mods due to this.