Next time someone asks me about how some aspect of American language has changed, I'm going to send them here: a 100 million word corpus, taken from Time Magazine from 1923 to 2007.

Of course I search "uh" (no hits) and "um," where I discovered this from a review of Britania Waives the Rules, from 1935:

In their paragraph on the speech of England's Best People. Authors Douglas & LeCocq disclose some of the secrets of its complex simplicity, consisting of " 'um's, 'aw's, and 'er's, the meanings of which vary according to the context. 'Um' may mean 'These are good tripe and onions.' 'You smell like a rose,' or 'Waiter, another whisky and soda.' This sort of thing makes it difficult for the foreigner, but the English themselves can tell instantly what is meant by the lack of inflection in the voice and the complete absence of expression on the face.

The corpus calculates frequencies by decade, which confirms the notion that the rhetoric of the verbatim has become more available: "um" appears 6 times in the 1920s (and all of these are faux Indian English) and remains under 18 times per decade until the 1990s, when they are used 32 times (mainly to represent speaking) and 35 times in the 2000s (and we're barely 3/4 of the way through the decade).