I cruise solo and did Greek isles/turkey last year out of Istanbul on celebrity constellation. As a solo, I get adopted by lots of people and have met fantastic people on cruise critic where I join private guide trips for each port.
I've found that RCCL has a younger demographic (people at the disco!) and recommend this to friends who are cruising. I was not a fan of NCL when I sailed them awhile ago, may give the Epic single rooms a go when they sail somewhere I haven't been to yet.