I only really use facebook and I don't think I would gain much from being open about tickling or feet on there. The most I'd do is like a generic 'tickling is fun' page, just so it's out there - might hint to other people that I like it in some way and you never know, we might find a common interest.
I did once 'like' one of these pointless, generic pages called something like "if you tickle my feet I'm not responsible for what happens to your face" - I also then invited friends to 'like' it and a number did, which was fun - but these pages are generally pointless because after that brief moment where you go "omg yeah i'm so like that, that's so funny" and click 'like', you then never interact with that page again.
I wouldn't join groups or anything specifically for tickling/feet fetish as I wouldn't want that 'fetish' angle to come across on my facebook, there's no real need for that when I can just come at it from a 'tickling is fun' angle - I do browse them sometimes too see if there's anyone on there I might know, but largely they are fakes or nutters or just people that I wouldn't have any reason to get i touch with.
I've occasionally seen friends post updates mentioning tickling/being tickled/ticklish etc and I always take the chance to comment, but again, not from a fetish point of view, just for fun.