You can manually add custom services to the drop-down menu. Custom services will appear at the top of the menu.

var a2a_config = a2a_config || {};
a2a_config.custom_services = [
	["Example dot com",
	 	"http://www.example.com/add?name=A2A_LINKNAME_ENC&url=A2A_LINKURL_ENC",
		"http://www.example.com/images/icon_16x16.png"
	],
	["Example dot net",
	 	"http://www.example.net/add?name=A2A_LINKNAME_ENC&url=A2A_LINKURL_ENC",
		"http://www.example.net/images/icon_16x16.png"
	],
	["Example dot org",
	 	"http://www.example.org/add?name=A2A_LINKNAME_ENC&url=A2A_LINKURL_ENC",
		"http://www.example.org/images/icon_16x16.png"
	]
];