If yuu want to add a further link to that list you would need to add some code to various files.
However, if you did not need the GOOGLE+ link you could convert that setting to read Instagram instead.
To do this go to the lang directory file found at...
klass/ lang/ en/ theme_klass.php
https://github.com/lmsace/klass/blob/v3.4/lang/en/theme_klass.php?L58-L60
leave the strings that are on the left as they are, but change the wording on the right, so that you change Google to read Instagram.
Save the file, then go to Site Administration > Languages > Language packs
Then select the button under the first column to update your language files.
Next go to the settings and enter your Instagram URL instead of Googles.
Mary