Apple gets a social messaging backplane for essentially free while Google struggles with building their own. Strategically this puts them ahead of GOOG and AMZN and on better footing with Facebook.
Apple has previously tried to do a deal with Facebook to presumably implement similar features - those negotiations weren't fruitful, perhaps a stroke of good luck for both Twitter (#2 in the space) and Apple (Twitter less likely to be a competitive threat to Apple).
In some respects this is dangerous for Twitter too. If Apple users get used to these features and use them appreciably, Apple may move to re-implement on their own platform a la iMessage (a replacement for carrier SMS). The smart guys at Twitter probably have a pretty good understanding of how this commodifies some of their platform and how to manage that over the long term.
/r