UNITED STATES (REUTERS) - Shares of Nike fell after tens of thousands of people used the hashtag #NikeBoycott on Twitter.

The calls for a boycott came after sports apparel maker decided to use former San Francisco 49ers quarterback Colin Kaepernick in an ad campaign for the 30th anniversary of its slogan "Just Do It."

Kaepernick was the first National Football League (NFL) player to kneel during the national anthem as a protest against racism.

The kneeling attracted criticism, including from US President Donald Trump, who said it was disrespectful to the American flag.

Still, many social media users approved Nike's stand on social issues.

Athletes like LeBron James and Kevin Durant expressed support, posting images of Kaepernick's Nike ad on their Instagram profiles.

Retail analyst Mary Epner, the chief executive officer of Mary Epner Analysis, said: "I think that Nike decided to align themselves with Kaepernick because they stand for the same things. They decided that they wanted to be on a platform of equality, and justice, and ethics. And I think that that's what they're counting on their customers wanting to be on the right side of."

She continues: "I think that they probably researched this the upside and the downside before they came to this decision. But, in the end, we're in very controversial times, and they decided that he was a good person to align themselves with."

Nike confirmed that Kaepernick was part of the campaign and called him "one of the inspirational athletes of his generation."

It did not immediately respond to requests for comment on the boycott hashtag on social media.