Claire Cain Miller, "Google Tweaks Algorithm to Push Down Low-Quality Sites", NYT 2/25/2011:
Google said Thursday that it had made a major change to its algorithm in an effort to improve the rankings of high-quality Web sites in its search results — and to reduce the visibility of low-quality sites. While the company did not say so explicitly, the change appears to be directed in part at so-called content farms like eHow and Answerbag, which generate articles based on popular search queries so they will rise to the top of the rankings and attract clicks.
This is obviously not the sense of tweak that the OED glosses as "To seize and pull sharply with a twisting movement; to pull at with a jerk; to twitch, wring, pluck; esp. to pull (a person) by the nose (or a person's nose) as a mark of contempt or insult", but rather the sense "To make fine adjustments to (a mechanism)".
For the "fine adjustments" sense, the OED's first citation is 1966
1966 Punch 16 Feb. 233/1 He has been running a Morris 1100 ‘tweaked so it'll do nearly 100’.
and involves adjustments that might be a bit beyond "fine"; but the Jargon File entry makes it clear that in recent digital parlance, tweaks really are minor adjustments:
To change slightly, usually in reference to a value. Also used synonymously with twiddle. If a program is almost correct, rather than figure out the precise problem you might just keep tweaking it until it works.
So I was taken aback by the contrast between the headline ("Google Tweaks Algorithm") and the lede ("Google … made a major change to its algorithm").
But maybe for some people, tweak has come to mean merely "modify", whether the modifications are minor or major? The number of web hits for "major tweak|tweaks" suggests that this is true.