NextDoor is a really good one, especially if you work together with other staff/volunteers to get a message out. You can literally blanket the community if there are enough people from different neighborhoods. I've also had success on Imgur, posting some super long stay dogs. Your audience on there isn't just local, so if you're able to adopt to people in different locations it could be helpful.
I had a Tumblr account where I used to post pics of adoptable pets when I volunteered at my local shelter. I worked it up to a decent number of followers, but even then it didn't really make much of a difference. I'd say Tumblr is probably a waste of time.
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