iMessage lets you message who you want, how you want. All Apple devices include the iMessage application to make messaging easy and streamlined.
At this time, Shift cannot support iMessage.
Apple does not have a web application version of iMessage, and because of that Shift cannot support iMessage.
iMessage on your Mac is a desktop application and not web-based. If Apple ever releases a web application of iMessage, we will surely include it in Shift's application suite immediately.
To learn more about iMessage, click here.