I'm actually putting together a panel on social media for cosplay for Costume Con, if you're going to that.
For cosplayers, I still think there is value in using DeviantART because you can share pictures with full credits in better quality. However, it's good to reshare the links to those uploads on other platforms. For cosplay, I think you will get the most reach on Facebook followed by Instagram. Instagram requires a picture or video for every post and users tend to gravitate to adding lots of hashtags, but fairly short descriptions. It's not great for updating about appearances and such. It also doesn't have the sharing features common to most other platforms, so it's better for directly reaching people who want to follow you but it's harder to expand your reach on there. Facebook has a lot of very active people and you can use a business page to manage your cosplay stuff, include links to events you're attending (or even create events), and encourage conversation and sharing. Twitter is good for broadcasting and you can get some extra reach from people retweeting -- but there is very little interaction.
Also, you might want to look at a social media management tool such as Hootsuite to manage your social media presence as you can crosspost to multiple platforms and schedule posts easily with that.
That's just a start, but I hope that helps