Facebook. I'm a member of quite a few groups there, many of them related to genealogy and species identification. I don't really develop/maintain relationships through Facebook. I use the account so that I can keep my relatives updated on my parents' health, as they both have cancer.
Twitter is a mess. I have one, but I don't really use it. I recently began posting my photos on Instagram because I like taking photos of flowers, plants, and insects. I use Tumblr, but not to socialize. I feel like that's the website to go to if you want to get doxxed.