[thaumasiotes]

Well, internationalized usernames are "available" on weibo in the sense that your displayed "name" can be anything you want. But you don't log in with your displayed name; it's an arbitrary bit of account data, and is changeable whenever you want. You log in with an email address, which is how the system identifies you.

(checking now just to make sure, I see that weibo allows three options for logging in: an email address (not internationalized), an account number (not internationalized), and a phone number (not internationalized))

[thaumasiotes]

I admit I don't understand the downvote. Email already has internationalization in the same sense as weibo does. You might receive email from me as 'From: Michael Watts <[e-mail]>'; the email address doesn't support arbitrary characters, but the name does (I've received email from '"=?gb18030?B?w8DIy7nY?=" <[e-mail]>', which worked out to a displayed name of 美人关). Similarly, if I wanted to display 美人关 as my handle on weibo, I could do that, but I wouldn't be able to use it to identify my account.