The appeal has been kept similar over the years, usually consisting of a message letting visitors know that Wikipedia is run entirely from donations, and pledging to never run ads. They mention they don’t often ask, and that if everyone were to donate a small amount, they’d quickly reach their target.
The Elaboration Likelihood Model (Petty & Cacioppo, 1986) describes two strategies of persuasion – a central route targeting logical thinking, and a peripheral route targeting more basic desires. Wikipedia’s appeal takes the central route, with a paragraph that asks for a little more thought to process than the average advert we have pushed in front of us nowadays. However they’ve done very well every year in donations, partly because those visiting the website are already in the frame of mind for putting more thought into what they’re reading than, say, on a bus home after an exhausting day. Wikipedia know their audience, and it shows in their support.
Petty, R. E., & Cacioppo, J. T. (1986). The elaboration likelihood model of persuasion. In Communication and persuasion (pp. 1-24). New York, NY: Springer.