The bike pictured, that broke at the seat post tube, looks like a very early and older model Montague bike. The company has been making folding 26" bikes for over 25 years, (and now 700c models), and the bike in the photo looks like one of the early 26" aluminum frame models. I own one of the more modern dual top tubes models.
I will be interested in watching to see if there are other instances of frame failures.