It shares a key with a "3"

• Key word in social media
• #StarringJohnCho, e.g
• Character in many a tweet
• Content-sharing convenience
• Twitter trend symbol
• Tweet symbol
• One of 140 characters, often
• Modern categorizing aid
• Word or phrase to aid online searches — hag's hat (anag)

Wiktionary Word definitions in Wiktionary
n. 1 (context Internet English) A metadata tag, signaled by a preceding hash sign, used to label content. 2 (context informal English) The hash sign itself. vb. (context transitive Internet English) To label (a message) with a hashtag.