Samuel Johnson's Dictionary (London, 1805) defines a commonplace book as a "book in which things to be remembered are ranged under general heads." He also lists a verb "to commonplace" which means "to reduce to general heads." Commonplacing is thus a very common activity, according to Johnson.
This already reflects a very watered down conception of the "common place," most likely influenced by John Locke's ideas. From antiquity to the Renaissance and early modernity, a common place was a "privileged place," not just any general heading that might or might not be arranged alphabetically. Loci were more like categories in which one searched for knowledge or on which one founded arguments. They usually constituted a kind of system, as I have argued before.
1. See Locke on commonplaces.
2. See also Search for commonplace.