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Has the rate of vocabulary change (that is, number of words falling out of use per decade, say) been found to be largely constant in human societies or does it strongly depend on circumstances?
If writing happens to be a major factor, has this rate been largely constant before the advent of writing?
Contra @JoFrhwld, it's not really that controversial to say that the rate of cognate replacement differs in different languages, or in different parts of the vocabulary. Glottochronology has a bad name in linguistics for exactly this reason: the assumption of constant rates of change made by glottochronology is clearly shown to be wrong when the methods are tried out on languages with known histories. Icelandic is a classic example: glottochronology calculates many fewer years separation between modern Icelandic and Old Norse than we know to be the case from the historical record.
I would recommend you actually read the article referred to in JoFrhwld's answer if you're interested in this: Frequency of word-use predicts rates of lexical evolution throughout Indo-European history (Nature) and also try Languages Evolve in Punctuational Bursts (Science). It's pretty lame to criticize a paper on the basis of your evaluation of the reputation of the journal or the disciplinary affiliation of the authors when you haven't even read the paper! These papers give a clear quantitative demonstration of a couple of the factors that influence the rates of language change, things which were only described qualitatively before.
As to societal differences, Lupyan and Dale give at least some hard data that language structure is partly determined by social structure Language Structure Is Partly Determined by Social Structure (PLoS ONE). There's good evidence that language change occurs faster in smaller populations, because it's easier for an innovation to diffuse across the entire population, exactly the same way that genetic innovations are more likely to fixate in small populations than big ones.
Instead of talking about words falling out of use, how about we talk about "cognate replacement" (there's a good search term for you). So, in English, we say to walk and in Italian they say ambulare. These two words have very different histories, so we'd call them different cognates. In English, we have a less commonly used verb to ambulate, which is cognate with Italian ambulare.
Now, let's take two more closely related languages, Italian and French. Italian's word for "to walk" is ambulare, and in French it's marcher. These two language are both descendent from Latin, which used ambulare for "to walk". So at some point, French replaced its word for "to walk" with another, historically unrelated word.
Whether or not this kind of cognate replacement happens at a constant rate is, I think, a rather controversial question in Historical Linguistics. It is at least as controversial as whether or not genetic mutations happen at a constant rate. There is at least one paper which argues that frequent words undergo cognate replacement at a slower rate than other words, namely Frequency of word-use predicts rates of lexical evolution throughout Indo-European history. I haven't read the paper closely myself, but note that 1) It's published in Nature, which has a bad reputation as far as language oriented papers go, and 2) None of the authors are linguists.
As for societal differences, I know of no hard data, but I would suspect that a well regulated literary culture would slow down cognate replacement, and that high rates of language contact/bilingualism would speed it up.
