Hi,
In Messages and iChat the answer is, eventually, yes.
My experiences is with iChat 2 through iChat 6 to Messages 8.
These can video using the AIM and Jabber services to other Mac users.
Whilst the video feed is based on what the processor can do, tied in with what internet speed the App "read" at the App start up and what settings the computer and the app have.
The app will try for a pic in iChat/Messages of 640 X 480 pixels. (about 0.3 Megapixels) (irrespective of the camera's abilities)
It will then try to aim for 30fps id the Processor can manage it.
It will drop the pic size and the frame rate to try an match not using more that 80% of the bandwidth you have.
In Messages (and iChat) you can call down the Connection Doctor during a Chat and see the current Frame rate and Data rates.
These can be effected by the setting in the Preferences > Video section about the Bandwidth.
As your ISP service varies the app will try an balance that against keep the video smooth (Video has the priority).
It accomplishes this by dropping frames. (at less than 10 fps the pic is too jerky).
There comes a limit to how much audio will go out of sync when you do this and at that point the audio gets dropped.
It seems FaceTime will do something similar although you don't have the equivalent Connection Doctor to check.
Technically FaceTime's Video is even more compressed as it may have to be managed by an iPhone at the other end.
I haven't run the same tests on FaceTime as I have with iChat on this.
There is also no adjustment to cope with really extreme fluctuations in the ISP service in FaceTime.
As an example:-
iChat and Messages needs just 256kbps up and down to have a 1-1 Video chat.
A 5% variance at these sorts of speeds is relatively small.
However if you have a 10Mbps download service and it varies by 5% this could be as much as 1500kbps (which is the effective top limit of iChat and Messages)
9:46 pm Tuesday; May 27, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad