For every website that switches from jQuery to Prototype, 8 make the switch in the other direction.

Changes in the usage of JavaScript libraries in 2012:
jQuery: +11.73%
Mootools: -0.33%
Prototype: -0.60%

75.6% of the websites that use the Prototype JavaScript library also use Script.aculo.us. 49.2% of them also use jQuery.

The last release of Prototype was 19 months ago. Its usage has fallen from 4.9% to 4.1% since then.

JavaScript libraries usage amongst the top 1000 sites: 1. jQuery, 2. YUI, 3. Prototype, 4. MooTools.