SAN FRANCISCO (Reuters) - Snap Inc shares tumbled below $20 on Thursday for the first time since the company's $3.4 billion public listing after the Snapchat owner received another "sell" rating from an analyst.<div><a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=wBs98uHU5wo:F59-DwWgW80:yIl2AUoC8zA"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?d=yIl2AUoC8zA" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=wBs98uHU5wo:F59-DwWgW80:V_sGLiPBpWU"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=wBs98uHU5wo:F59-DwWgW80:V_sGLiPBpWU" border="0" /></a> <a href="http://feeds.reuters.com/~ff/reuters/technologyNews?a=wBs98uHU5wo:F59-DwWgW80:-BTjWOF_DHI"><img src="http://feeds.feedburner.com/~ff/reuters/technologyNews?i=wBs98uHU5wo:F59-DwWgW80:-BTjWOF_DHI" border="0" /></a></div><img src="http://feeds.feedburner.com/~r/reuters/technologyNews/~4/wBs98uHU5wo" height="1" width="1" alt="" />