hey william,
you can find this string at yourjoomla/language/en-GB/en-GB.com_easysocial.ini

having that said instead of manually editing the file I'd recommend to use joomla language overrides, otherwise you would need to modify the file again after every update. you can find more about language overrides here: http://stackideas.com/blog/how-to-override-joomla-language-strings

hope that helps and have a nice day