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Would these technically be competitors?

Steem.io is supossed to be about the Steem blockchain not Steemit.com. In that context they are not competitors. There should be a list of all the major apps conecting to the blockchain divided in categories (e.g. social media sites, wallets, tools, etc)

That depends on their particular reveneu/investment/'charity' model. I think currently some of them are, to a certain degree, competing.

Theoretically, although I don't want to imply that this is necessarily the case, Steemit inc (whom I suppose controll the Steem.io site) could of course deny listing any apps that they think could be hurting their bottom line. At this point in time however , I have a hard time seeing why they would.