I don't think it's fair to compare those two. One is a while it's still happening tweet, the other is a post mortem from after the incident.
Tumblr's engineers are likely very busy right now working on getting things back up and running. You can't expect them to take time out from that to write a detailed incident report for the public. Wait and see what they release tomorrow before making a judgement.
Simply put, what mcdavis said. GitHub also has a history of being communicative during issues; not to mention near-instantaneous fixes of problems (anybody remember how unbelievably quickly they re-architected their entire site to deal with the scaling and speed problems that had barely started to surface? how about the lack of speed issues since?) as opposed to these things I’m hearing of people seeing the same, obvious, bugs over the last two years or so.
Tumblr's engineers are likely very busy right now working on getting things back up and running. You can't expect them to take time out from that to write a detailed incident report for the public. Wait and see what they release tomorrow before making a judgement.