The name Robby ranked 780th in popularity for males of all ages in a sample of the 1990 US Census.
Though this name appears on the 1990 U.S. Census lists, it is used by only a small percentage of the general population.
When the Norman French stormed the shores of England in 1066, many Roberts were numbered among them. The defending Anglo-Saxon army no doubt contained some who were known by that French name's Germanic source, Hreodboerht. The Normans were victors on the field, and the Germanic name likewise gave way to the French.
In fact, Robert became such a popular name that many nicknames evolved to help distinguish all those Roberts. Two of these survive only in modern surnames: Hob gave us Hobson, Hopkins and Hobbs, while Dob was the source of Dobson. Nob was another nickname for Robert, now completely out of use.