In the lead up to The 3rd Birthday's release, the game's official Twitter account was one of our main sources for information. Square Enix has resurrected that account, now as an official PR account for the 1st Production Department. Access it via @1stPD_PR.
Square Enix's 1st Production Department is the division that makes the company's biggest Japanese games -- things like Kingdom Hearts and Final Fantasy XIII.
The 1stPD_PR feed says it will relay event information and notices about The 1st Production Department. In general, development staff will not appear at the Twitter feed, so we won't be getting responses to questions about game content (which is really unfortunate because this was one of the main attractions of the 3rd Birthday Twitter).
It seems that Square Enix somehow managed to change the username on The 3rd Birthday account to 1stPD_PR, so if you already subscribed to The 3rd Birthday Twitter's RSS, you'll be subscribed to the 1stPD_PR Twitter.