I'm kind of surprised that whoever owns Tumblr this week isn't pushing it harder; Tumblr is, when it comes to it, more or less twitter without the character limit and with a much worse mobile app (most Twitter features were effectively copied from Tumblr).
That said, I'm not sure that there needs to be a single direct replacement for Twitter; historically, when a social network dies, generally it isn't replaced by a direct clone but by different things.
I'm kind of surprised that whoever owns Tumblr this week isn't pushing it harder; Tumblr is, when it comes to it, more or less twitter without the character limit and with a much worse mobile app (most Twitter features were effectively copied from Tumblr).
That said, I'm not sure that there needs to be a single direct replacement for Twitter; historically, when a social network dies, generally it isn't replaced by a direct clone but by different things.