The Oxford Thesaurus of English says that the noun know-all is informal. For example, when saying:
You are such a know-all.
My concern is with using know all as a combination of the verb to know + all. Example:
People today know all about technology-use.
Is doing this informal or conversational in this sentences, because we can have more formal alternatives such as more knowledgeable, more experienced, more familiar, and so on and so forth?
Know-it-all or know-all is an idiom which occasionally used in a positive sense to indicate that the person is indeed a wealth of knowledge, but is most often used in the sense that the individual thinks he's an expert on everything and will, if permitted, spend hours bending your ear to explain how he's right and you're wrong.
To "know all about" a topic is generally a literal statement (though perhaps a hair hyperbolic) -- the individual is an expert on the topic.
In the example sentence
People today know all about technology use.
Know all is not a verb. Know is the verb here; all is part of the object of the verb.
People today know.
What do they know?
People today know all about technology.
Note that all isn't being added to the verb. It is a modifier of the object. Consider this:
People know about technology.
How much do they know about it?
People know all about technology.
All describes how much they know about technology. It has nothing to do with the verb. Even without the tech bit, I know all is a simple (but valid) sentence with a subject (I), verb (know), and object (all). It could be I know Bobby or I know French or I know a secret and it's all the same: I know [something]. In this case, it's I know [all about technology].
Meanwhile...
The noun know-all (or know-it-all as is more common around here) is an informal term because it's a mashup of two (or three) words to describe someone in a less than flattering way. A formal version (without the sarcasm that goes along with know-it-all) would be subject matter expert (or just expert).
So, to sum up, if you want to say people today know all about technology use, that's perfectly fine in both an informal/conversational or formal situation. If you want to acknowledge that someone knows what they're talking about in a more formal setting (without being derogatory), I wouldn't use know-it-all.
