Could you let me know a specific page you're adding the social login buttons?
If you're using the following:
* SCLogin, there are 2 language strings that can set a pre-text or post-text above the social login buttons - MOD_SCLOGIN_SOCIAL_INTRO_TEXT_LABEL and MOD_SCLOGIN_SOCIAL_POST_TEXT_LABEL
* Joomla's login page with the JFBConnect > Configuration > General > Integrate into Joomla Login option, we should be adding pre-text with the language string COM_JFBCONNECT_LOGIN_WITH
* If you're using the {JFBCLogin} easy-tag, there's a parameter to pass in called 'text', so you could do {JFBCLogin text=Login With}
Otherwise, let us know how you're adding the social login buttons so we can give you a suggestion.
-Melissa