Using XFBML, you could change the text of the login button as follows:
<fb:login-button>
Custom login text
</fb:login-button>
This is also possible similar to the HTML5 version of the button:
<div class="fb-login-button">
Custom login text
</div>
The problem is that, unlike XFBML, which is displayed after loading the SDK, the HTML5 version is displayed by the browser even before the FB SDK is initialized, as a result of which the usual line "User login text" appears in the login place before the SDK initializes it and change it to the look of Facebook (something like FOUC - a flash of loose content).
Is there a way to specify custom text and avoid this problem? For instance. some data parameter or any JS method how to change text dynamically.