Yea, 3speak is using HIVE chain only. I think you subscribe with email (guess to make it more easy for users to start using 3speak not having a HIVE account) and when you have your account, you can connect HIVE account. Though with D.TUBE you can also post to the HIVE chain, they still stick with STEEM as well; Something I really dont like since this downgraded the offering of D.TUBE to just another centralised platform. That said, I think they gonna launch their own blockchain, but have no idea how centralised or decentralised that is, never looked into it, and probably will not do as well. On top of that, I opened many D.TUBE STEEM/HIVE posts and noticed that quite a few times the videos are not playing, ie bad implementation, low quality of service, while with 3speak I never had a problem so far.