Feeling lonely at work is, it seems, not uncommon - even if you're not a solo freelancer like I am. (Funnily, I don't feel lonely - there's usually so much contact with other people via mail, and enough interaction with customers and colleagues, that I don't suffer from this.)
Because feeling lonely is not very nice and not very healthy, the Institute for Life at Work is doing a survey about workplace loneliness. If you're interested, you can read more about their work here, and take the survey about your own work loneliness scale here.