$params->get('fb_moduleviewset');

here fb_moduleviewset is the parameter name which you
set in module's main xml file.this parameter
value is set in mdoule settings in joomla admin side
which you know well.

this is how i get some more parameters value -

$fb_postcolor = $params->get('fb_postcolor');
$fb_backcolor = $params->get('fb_backcolor');
$fb_backcolorcom = $params->get('fb_backcolorcom');
$fb_colorcom = $params->get('fb_colorcom');

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