I have a single bunny who sadly has to live outside - she came to me as a single rabbit and we've tried for over a year to find her a friend but it's not worked out. She definitely seems alot happier without a friend as she's really really aggressive when other rabbits are anywhere near her. I think having a single rabbit living outside is more time consuming as you really have to spend a lot of time with them every day.
My bonded pair seem fairly happy although they have little tiffs sometimes, but the female doesn't like humans so I know she is happy having a rabbit partner to chase around.