We have spent a long time making sure that every nimby we can get our hands on knows an immense amount about their backyard, we feel that local support is THE critical factor to sustaining our campaign, that it is ultimately local people who will protect these sites and it is the future of local people that is most important to the future of these sites.
I also feel that by adopting a site that is under threat as your own backyard (even if it isn't) will put you in the mindset for continued action.
And it should be just the one site - split your activities and you massively dilute your effectiveness. I'm not saying you should not care of course, or help out with other sites, just that ten individuals paired up with ten sites can achieve more than 100 people sharing the same sites.
Rather than adopting, I think of it as fostering. At some stage some authority will come along and say "it's mine now" - EH or some such. At that point it's probably better to move on, even though I'm sure it would be possible to see problems with their approach or see how I could do it better, the fact is I'd be stressing with EH whilst other sites were being trashed.