Buttons is a good way, hide/show meter on mouseover is ok, but makes for a lot of code. Another alternative is ImageTint
[Rainmeter]
Update=500
MouseLeaveAction=!RainmeterSetVariable FaceBookColor 255,255,255,255
[Variables]
FaceBookColor=255,255,255,255
[MeterFaceBook]
Meter=Image
ImageName=FaceBook.jpg
GreyScale=0
ImageTint=#FaceBookColor#
DynamicVariables=1
MouseOverAction=!RainmeterSetVariable FaceBookColor 215,221,253,255
LeftMouseUpAction=!Execute ["whatever"]

Artifacts are from the crappy .gif format, not ImageTint
More dramatic results can be had by using more dramatic color differences, and can be really striking if you use GreyScale=1 which in effect makes ImageTint "colorize" the image instead of "tinting" it.
Doesn't let you have two different images of course, but can give a nice hover effect with a minimum of code. Button meter is really the most efficient, but does require some Photoshop/gimp work to create the bitmaps.