Some people have noticed that the search bar on facebook looks like this, and suggested an improvement:
They think it should read 'Search for nouns'.
I'm not having a go at these people; the OED defines 'noun' as follows:
A word used as the name or designation of a person, place, or thing.
Wikipedia sensibly adds or idea to its definition, however, as liberty and disappointment are both nouns, and further down expands this to:
person, place, thing, event, substance, quality, quantity, or idea, etc.
Because Wikipedia is often written by experts, it's a lot more detailed than the OED and goes on to tell us that this definition is not very useful on account of it's a bit fuzzy and doesn't tell us much, and that it's better to have a formal definition based on the properties of the elements we class as nouns.
One way to do this is to use such characteristics as 'can occur with a determiner like a or the', or 'can take plural inflection -s'. These characteristics are often not cross-linguistic, but can help to identify nouns if you know the rules for the languages you're interested in. In beginning syntax classes I hand out a cheat sheet with just such tests.
As I think I've mentioned before, the distinguishing characteristic of nouns that linguists generally use is if they behave like nouns. Does that sound circular? What I mean is, the name 'noun' is used to refer to a class of words that all behave in the same way. They can be the subject of a sentence, for instance, or the object of a verb or preposition. These are grammatical characteristics. The 'people, places and things' definition is a semantic definition: it describes what nouns mean or refer to.
We can use both to identify nouns, but the semantic definition is not appropriate to denote what facebook wishes you to search for; it does not think you should search in that box for love, peace and understanding; it will not bring you success (or at least, not as an abstract noun).