We started out with 3 gerbils, but for the last couple of months we're down to one (he's about 3 1/4 years old) and I think he's pretty lonely. Unfortunately he was the one of the three who most disliked human handling, and so i'm not sure he super enjoys when we take him out of his enclosure.
He's got his wheel and a huge enclosure all to himself, but he's been noticeably less active since the others passed.
I always wonder if our pets think we're eternal beings or something, I've had my dog since she was a pup and now she's old and i (from her perspective) have hardly changed.