It's safe to use. The previous implementation of createPost() returned void, so you aren't already assigning its return value to anything. If anyone continues to not assign it to anything then no problems...but now you have the option of assigning it to a java.net.URI if you're interested.
Snapshots are generally safe to use, although there are no guarantees. This is especially true of snapshots that are pre-1.0 (as is the case with LinkedIn), as we reserve the right to change our minds. But I avoid making breaking changes unless I've thought it through and can think of a darn good reason to do so.
And, BTW - Only god knows why {insert any public API provider here} designed their API's in such a way...
Craig Walls
Spring Social Project Lead