If this feature was in Mountain Lion Developer Previews prior to revision 4, I didn’t notice it, but you can now tweet directly from the Notification Center “pane”.
You first need to make sure said feature is enabled in System Preferences (note the use of the “old” Notification Center icon in the preference pane icon set…perhaps Apple will change that prior to the final Mountain Lion build):
If it is enabled, you now have a Twitter – er – “button” [?] in the Notification Center pane, and clicking it gives you a tweet box where you can drop your 140 from all your connected Twitter accounts.
I suspect there will be similar Facebook posting features integrated, but I’m not part of the Facebook zombie horde and have no way of testing it.
I also noticed that Chrome now has tighter integration with the Notification Center than it did before (as in, I actually saw notifications pop up from Google Mail) and both the Beta channel and Canary builds have integrated the functionality: