This is a grave from the Victorian age when a fear of zombies and vampires was prevalent. The cage was intended to trap the undead just in case the corpse reanimated.
Victorian-era coffins were enclosed in cages to prevent zombies from escaping.
Hoax or Fact:
The picture messages show a grave enclosed with a cage like contraption, claiming that the grave belongs to Victorian era and the enclosed cage is meant to trap the zombies, just in case the dead people came alive again. Although it might sound interesting, the message is not a fact. The cage over the grave is a 'Mortsafe', which is only meant to protect the dead bodies from being stolen.
In early 19th century, grave robbing became a growing concern, especially in Scotland, as robbers used to steal the dead bodies and supply the corpses to medical students, for dissection studies. This is how the idea of contraption cages over graves came up - to protect the dead bodies from being stolen, and not to trap or prevent the undead zombies from escaping. These Mortsafes were either cages or grills placed over the graves of newly deceased people during the 19th century, and were made of varying materials, typically from iron or stone.
About Source of the Picture
The source of the picture in question is a grave in Woolpit in Suffolk of England, shot by a photographer named Richie Wisbey. He has shared it on his Flickr account - making a joke about zombies. Later he even admitted that it was not meant for zombies, but to stop body snatchers.
Therefore, the messages are simply hoax, in fact just a prank around zombies.