Object Oriented JavaScript optimization
One comment I might suggest is your hard reference to mass_message, consider passing that in as part of your event data, as it makes the code more re-usable (Several instances of your editor on a page?)

A thought to consider is to swap your hard b and i tags to instead define css classes to be used? This will give you code re-use, if you had bold and italic defined in a css, you could use the above code you have to simply change the class on click instead, allowing you to do more jazzy stuff, borders and such, or be very clever and have specific things you define, maybe floats and margins/padding etc. You could use span as a first try and set the class of the span equal to the specified format :)