I would like to believe that she left on her own, and as Meems has pointed out, there were Facebook posts about leaving.
However, the fact that her wallet, shoes, keys, cellphone, and dog were left behind suggests that she did not leave voluntarily. Further, she had a series of confrontations that night and may have been under the influence given that she visited two different bars that night. While she may have felt an urge to leave given different stressors in her life, I doubt that she would have vanished the way she did, almost without a trace, leaving in the middle of the night without any of the above items.
There's also the fact that the family is in the dark about where she is. From what we have heard, she was extremely close with her family and would have contacted them somehow. If she has contacted her parents somehow, they would have to be excellent actors to go on news shows, give interviews, and post things online while trying to find her. I am not at all critical of her parents, I think they have done everything they can to find her, they are still actively looking for her, and they have no idea where she is. Everyone reacts differently, but the Mel Gibson, Kevin Bacon, and Liam Neeson reactions are just in the movies.
In my opinion it is extremely unlikely, if not impossible, for her to have left on her own under these circumstances.
However this is just my opinion. At this point I don't think any possibility can be ruled out, and as time goes on with no new developments, I think a voluntary disappearance should be considered along with every other possibility.
Still, in my opinion, it is still far more likely that the guilty parties & possible explanations have already been discussed in the press, internally by LE, and/or on message boards and that people with information about what happened have held it together somehow and have not come forward to the police. This could change if anyone catches a new case and wants to makes a deal. Or if someone's conscience reawakens and they want to try to make things right in some small way. If anyone out there is following these posts, please consider reporting what you know anonymously, if for no other reason than to give her family closure or to be able to sleep easier. If greed is a more powerful motivator, there's also the reward to think about.
Just my opinion folks, not trying to offend anyone.