This forum is the best place for Quicksand general community but yeah I've been dipping my toes in the other social networks...
Facebook - they will automatically close down any alternate accounts you make. If someone used their main identity to create a Facebook "Quicksand Group" I understand it could be tricky (but possible) to hide participation in the group from the rest of your social network. Too much risk and trouble!
Twitter - I made an account to socialize and promote my artwork (@silkyfriction) but I don't actually post much. There's only a small trickle of wet-and-messy fetish community on twitter. You can check various hash tags, but stuff like #quicksand is a complete non-starter (too many people using quicksand as a metaphor, and talking about the band Quicksand). You really have to find individuals you like, and follow them, which seems impossible for quicksand interest because I don't think anyone's on there (and I don't count because I don't post anything)

Tumblr - Many artists and photographers like Tumblr because its a micro-blog that puts images front and center, it's easy to search for images by tag so I view it as a good place to browse images, it's not so easy to actually communicate with people though, there is not even an easy "reply" or "comments" function. Quicksand community veteran Kaol is on there, and he regularly posts awesome vintage, rare, and hard to find pictures of women-in-peril / jungle girl situations
DeviantArt - some good quicksand photographers and artists post there stuff here, including a large number of Viridian's awesome quicksand stories, it's easy to "watch" who you like and receive updates on their newest postings. Overall great, with lots of features.
Pinterest - don't even know