It can also sustain one that already exists over a long distance or time apart. Or it can add on to an existing relationship. For instance, I reconnected with a high school friend on social media 20 years after we graduated to find that she is a teacher looking for freelance work. She is now my freelance text editor.
The point is that social media can spark or add on to a relationship — either end — but it won’t be the entirety of it. Oh maybe the rare, pen pal kind of thing, but one can’t build a neighborhood from that.