I've gotten #2 on other sites. At first it was the most confusing thing. I basically don't even art, and these people were finding old submissions and faving them. My major guess is that they're trying to drive up traffic to their page, but even that doesn't make much sense.
I've done #4 with comments. Commenting (positively!) on things really deep in the archives of someone's gallery is just kind of fun to me.
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GioGio
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