I run the sites uktrail.com and small-hostels.com, which both provide directories of hostels. I gear the sites particularly towards solo travellers, because I have experience of being one, and because one of the biggest merits of hostels (as opposed to guest houses and hotels) is that they do not penalise single travellers relative to couples or groups.
I have had a request for an entry on small-hostels.com by a person who runs apartments in a city in Romania. He tries to be persuasive, saying that his prices in studios and apartments are as good as hostels. I will solve the question by giving him a mention on my Romania page, but not actually entering his property as a small-hostel.
When I asked the price for one person for one night he made the following comment:
"The price per person for 1 night is 15 €
But as we know in general people come on travel or in holiday on small group between 2-6.
In 10 years i have just 24 persons that travel alone."
I feel rather sad about this. But perhaps it is the other way round: BECAUSE he has apartments and private rooms, single travellers tend not to go there.
My impression is that the accommodations are very good and good value, with cooking facilities, but the sociability for a single traveller may be nil.