"The lines have been drawn," says Allie Townsend in TIME. In unveiling a new mobile operating system on Monday, Apple announced a tight integration of Twitter that will let users directly post to the microblogging service from a wide range of apps on their iPhones, iPads, and iPods, without ever having to open a Twitter-specific application. Facebook did not get similar treatment. The announcement clarifies some tech allies and adversaries: Apple and Twitter on one side, and Microsoft and Facebook on the other. But why would Apple "snub" Facebook, the largest social networking site, and opt to partner instead with a service that has about half as many members? Here, three theories:

1. Apple and Facebook have sparred before
Last September, Apple launched a music social network called Ping, which, at first, had Facebook tie-ins. But a "disagreement between the two companies" over the details of Ping led to Facebook being removed, "much to the detriment of Ping's success," says Jennifer Van Grove at Mashable.

2. Both companies insist on control
The obstacle to getting a partnership in place may have been "Apple's desire to maintain control over the user experience and preserve its direct relationship with its customers" — goals that "clashed with Facebook's own ambitions," says Bianca Bosker at The Huffington Post. Indeed, Facebook also wants to control the connection to users, says NDTV, and Twitter, by contrast, can be more easily integrated “as an underlying technology.” That gave Twitter the edge.

3. Facebook already friended Microsoft
Apple's longtime nemesis bought into Facebook early, and has integrated the social networking service into its own mobile operating system. That means "Apple was bound to be looking for alternatives," says NDTV. The relationship between Microsoft and Facebook means that anyone hoping for a similar deal between Apple and Facebook “will likely be left waiting for quite a while,” says Townsend in TIME.