What would you recommend for someone who has isolating hobbies, e.g. reading/writing?
Even if something is an isolated hobby, I think there are usually enough related activities for someone to get by.
For reading/writing:
book clubs
go to the library a lot and eventually you'll know the regulars
write in a social setting (coffee shop, park, ...)
writing club (never actually heard of one but seems like a thing that would exist)
volunteer to support literary things
poetry slams
there's probably some social aspect to NaNoWriMo if you look for it
take classes (creative writing, literary analysis, journalism, ...)
teach classes
and many more!
You probably won't find many places to just read or write silently in a circle of people, but there are plenty of options to talk about or do things with the reading/writing you do on your own time.