Only one of my aupairs went to English college. I require aupairs with good English and hire accordingly. Whether or not they go out a lot depends on their personality. There is facebook and lots of ways for aupairs to get in touch with each other that does not require them to attend a course. It is whether they want to. As fluffy says, some already have their own friends.
I think, as fluffy says, they need to have clear goals, but that doesn't have to be language classes. There are a million ways to meet people and as long as they can tell you what they plan to do while they're here I don't see why not going to English class is a problem. A lot of Scandinavian people have very good English, a lot of them also make very good au pairs. It would be crazy to dismiss them all because they can already speak English!
This is only my second year of getting au pairs so some things are new to me. Better English is nicer for my ds but I've read threads about au pairs who get home sick or clingy because they have no other friends. I love to chat but some evenings after dinner and putting my child to bed I have to study/work. So won't always be chatty. I think I'll follow the tips on here about asking them what they want to do with their time here.
One of my au pairs didn't go to school, she was German but with an English dad and spoke excellent english. I was reluctant when I interviewed her for all the reasons you have listed, but agree with others, ask her what she plans on doing. Mine was interested in photography, guitar and art, so I researched some courses at our local adult ed college which she signed up for. I arranged meet ups with other friends with au pairs, and she soon had a ready made social circle.
She was fantastic, my favourite of all of our au pairs, so don't let it put you off if she seems good.
I insist non-fluent English speakers do an English course. It gives them a purpose for being here, shows commitment and also makes them less likely to leave at short notice (if they've paid for a half term ahead).
It also gets then out of the house and meeting people.
Be upfront with potential au pairs if you have concerns about them making friends when they don't attend language classes. They may already have friends in the area or ideas of other classes they can take to meet people. If you get the feeling that they haven't thought about what they will do with their free time or how they will make friends, maybe you want to pass on to another candidate.
With regards to having some time for yourself, same applies. Let them know that they are welcome to spend time with you evenings and weekends but they you are supplying them with their own TV, access to WiFi or whatever and that you need some time to yourself. They can decide if they would be happy with that, it's always easier to be upfront about these things at this stage rather than when the au pair is actually sitting on your sofa hogging the remote control!
None of my au pairs have done college courses as they were already fluent. One tried to sign up but was told her English was too good and advised her to try Uni! Both of mine have been keen Gym bunnies so have gone running and to the gym, had a huge circle of friends and been very active in the village, there are ample fun courses to do, volounteering work etc. I have never heard of a boored Au pair here yet!
I've only ever had one au pair who went to language school and they have all managed to find a social life, often through the church. A lot of the Eastern European girls come over as a first step to staying permanently so although they do want to learn English they aren't in a rush.
My au pair goes to the gym alot, and is part of climbing and running groups as well as having English lessons. She says these are all good social activities (our local English lessons are not that good for socialising as most people are not in her age range).