Honestly there really just is nothing like interacting with another person right in front of you, not forums, instant messaging, phone calls or webcam chatting that can replace the physical interaction. I feel like your brain is hardwired to approach these forms of communication differently, unconsciously, consciously yadda yadda.
As for loneliness, its like the physical presence and enjoying it with those persons you connect with that absolutely cannot be replaced. I am one of those people that feel lonely, I have my parents, my brother, and pets but a friend? Well thats complicated. my best friend lives in another state, how? Well, I met him online, but hes the only one that I've ever really clicked with as a pal. I'll admit I was horrible at interacting with others (my awkward stage) my freshmen year of high school, eventually I became more well spoken. I also did stand out, I wore clothing to my style ("where did you buy such great shirts" "your shoes are really cool"), and I am one of those guys that cuts and styles his own hair (haha...ha). I only managed to become this guy because my anxiety stems from new situations, things I cannot foresee, unexpected social interaction. With school, 5 days a week its easy for me to get comfortable and express myself.
Yet, I felt disconnected, I was always too anxious to invite a friend over, go to their house, go to that concert, go to that party etc the list goes on. High school friends for many people are just friends you feel obligated to talk to or hang out with because you see them every day. In fact you interact so much with them you meet outside of school, but they still just become "school friends" in the long run, you graduate and poof they're gone. Maybe you'll be lucky and make that friend more than just an "obligation", maybe. The former however, was my situation, except.. I gave up 9/10 of the social activities with those "school friends" because of my anxiety. This made me feel incredibly lonely, I didn't give a chance for those relationships to blossom, I spent nights alone wondering "what if". I graduated, I still talk to one high school friend.
The loneliness comes from not feeling those connections, feeling misunderstood. Yeah I have my parents, I have my siblings, I have those (meagor amount of) friends but they don't quite understand me like I'd like them to. The loneliness comes here, it moves into that gap that is unfilled.