I do not believe "etc." in this case (perhaps in any case) should fall prey to serial comma conventions. In order to do so, as you note in your example, there would need to be an "and" before it, which doesn't really go with "etc.". If you simply took the comma out it would read (I'm paraphrasing, I don't have the exact words on hand), "using Twitter, posting on Facebook etc." I feel in this case that using the comma is correct regardless of serial comma conventions.

The serial comma is definitely not a matter of grammatical law, as you said. As long as you either consistently use it or consistently don't use it, you're fine. The standard convention even differs from country to country. In Canadian grammar, for example, it is customary to include the serial comma between the last two items of lists. Bread, milk, and cheese. I'm sort of fond of the little bugger myself.