Always read the primary source is my motto:

   The first case, Knotts, upheld against Fourth Amendment 
   challenge the use of a “beeper” that had been placed in 
   a container of chloroform, allowing law enforcement to 
   monitor the location of the container. 460 U. S., at 278. 
   We said that there had been no infringement of Knotts’ 
   reasonable expectation of pri- vacy since the information 
   obtained—the location of the automobile carrying the 
   container on public roads, and the location of the off-loaded 
   container in open fields near Knotts’ cabin—had been 
   voluntarily conveyed to the pub- lic.6	Id., at 281–282. 
   But as we have discussed, the Katz reasonable-expectation-
   of-privacy test has been added to, not substituted for, 
   the common-law trespassory test. The holding in Knotts 
   addressed only the former, since the latter was not at 
   issue. The beeper had been placed in the container before 
   it came into Knotts’ possession, with the consent of the 
   then-owner. 460 U. S., at 278. Knotts did not challenge 
   that installation, and we specifically de- clined to consider 
   its effect on the Fourth Amendment analysis. Id., at 279, n. 
   Knotts would be relevant, per- haps, if the Government were 
   making the argument that what would otherwise be an 
   unconstitutional search is not such where it produces only 
   public information. The Government does not make that 
   argument, and we know of no case that would support it.

I think I finally understand why Schmidt, et. al., have gone on record (while clearly smart enough to know the ire that it would raise in the geek community) that '[we] do not have a reasonable expectation of privacy' when using their systems.