Facebook has blocked Path's access to its social graph due to Path's recent spam debacle. This will prevent Path from being able to access the social network's "Find Friends" feature, meaning the app won't be able to spam your contacts list with invites. However, users will still be able to login to Path via Facebook and they will still be able to share their content onto their walls.
Earlier this week, Path was in boiling water after many users complained that the app was spamming text messages to all of their friends. The text messages were sent around 6:00A.M. last week, disrupting a lot of people's sleep. After a series of complaints, Facebook decided that it's in everyone's best interest if the service no longer had access to their friend's personal information.
Unfortunately for Path, losing access to Facebook's social graph will be detrimental to its growing user base. It was just last week when Path announced that over 10 million users have registered for its service. It may have trouble reaching its next milestone if it doesn't find another way to entice users fast (without the need of spam of course). It can still let people invite others through their Gmail accounts, contact books, and Twitter accounts, but even then, it still may not generate nearly as many new additions as when it was able to let users "Find Friends" on Facebook.
Path isn't the only service to be banned from Twitter. Facebook has also blocked MessageMe's access to its social graph, as well as Voxers. It's speculated that Facebook blocked MessageMe because it poses a threat to Facebook's own Messenger service, however Facebook stated that it was because the service copied Messenger's core functionalities. Facebook's social graph is important in helping these apps obtain a huge user base, so being forbidden access to it can spell danger for their futures.