Bitly has announced that they are limiting the number of branded URLs on free accounts to just 500 links per month.
This limit was much higher until now. The company said in a note:
For 99% of our personal users, 500 branded Bitlinks each month is more than enough for their efforts. If you need to create more than 500 branded Bitlinks each month, please contact us about upgrading you to one of our commercial packages.
If you cross the limit, newer links would use the default bit.ly branding for the rest of the month.