No, of course, that is very true. Like you, I've no idea how many social tenants buy a home on the open market. Instinctively I'd imagine that it would be relatively low, hence why right to buy schemes exist to try and make it easier, but I have no stats to back that up.
I suspect I don't know enough about the matter to have an informed opinion either way. If you're asking therefore if I think social housing should exist or not, my honest answer to you would be "I don't know".
I always imagined social housing was for those in dire straights financially rather than those with well paying jobs. That it's delivered at rents that are below market rates makes it a form of welfare imo.
The above was informed via Shelter by the way. If they haven't got the whole picture then of course, do please add some further context.
http://england.shelter.org.uk/campa...proving_social_housing/what_is_social_housing