A lot of women do find loners attractive because keeping to yourself gives you that element of mystery. It also makes a woman think that maybe because you don't participate in a lot of social yammer, you think more and are more sensitive. My husband is like that and it's one of the qualities I love about him. I wouldn't call him a loner necessarily, he is NOT anti-social, he's a really nice, friendly person, but he does keep to himself somewhat and doesn't seek out the pointless gossip and cliques. A benefit of a "loner" is that his lady often winds up also being his best friend and then she gets most of his private attentions instead of him flitting off to be some sort of social butterfly.
Not really... You look for someone who has a good camaraderie within their circle of friends. No friends? Red flag. (my first thought was: when they interview the neighbors of a serial killer, they always say "he was a good neighbor, very quiet, kept to himself...") it's important to our well being to have some kind of social network... It's not like you have to be together all the time, but deep down, we all need to feel loved and cared about!