I have to confess, I am always slightly surprised by responses such as from the first poster, as I always forget that for some people relocation is a 'big deal.' Whereas I have always considered relocation to be a normal part of life.
I can understand your predicament, and it's good you've looked at practical solutions such as volunteering, joining clubs etc.
What time does your working day finish (at the latest)? There are plenty of volunteering opportunities which require evenings and weekends only. And a fair number of options allow some degree of flexibility- you are, after all, volunteering your time freely.
As for clubs and societies, running clubs tend to be very social and also cheap (I pay less than £20 for a whole year's membership). Plus there are obvious health advantages.
Your housing situation can be solved by looking for a more sociable houseshare. Obviously, people come and go, but a houseshare which is sociable now is more likely to attract friendly, sociable people in future- particularly if your landlord lets you vet future tenants (as most do- it is in their best interests to maintain a happy household).
Moving back home, given that you are happy in your job, would to my mind be unjustified in this situation and a mistake.