Everyone is logged into an account + Tweets (and share buttons) embedded all around the web -> way more data for advertising.
That's the underlying fact that make Facebook and Google (and to a lesser degree, Pinterest) so valuable for advertising. Facebook and Google have organic signups (FB for FB, Google for Gmail, Youtube, etc.), but Pinterest does what Twitter's thinking of doing here--it locks you out of content until you have an account. Pinterest also has "share" buttons (and the accompanying JS, cookies, etc.)
That's the underlying fact that make Facebook and Google (and to a lesser degree, Pinterest) so valuable for advertising. Facebook and Google have organic signups (FB for FB, Google for Gmail, Youtube, etc.), but Pinterest does what Twitter's thinking of doing here--it locks you out of content until you have an account. Pinterest also has "share" buttons (and the accompanying JS, cookies, etc.)