get_the_author_meta()
returns the data for use in PHP. To display the
information instead, usethe_author_meta()
http://codex.wordpress.org/Function_Reference/get_the_author_meta
Your code does not echo
data even if it is present. Add an echo
or use the_author_meta
<?php if(get_the_author_meta('twitter') ): ?>
<a href="https://www.twitter.com/<?php echo get_the_author_meta('twitter'); ?>" target="_blank">Twitter</a></p>
<?php else: ?>
<strong>No website</strong>
<?php endif; ?>