SAN FRANCISCO — Ryan Knight, a Democratic activist in Los Angeles, called for a boycott of Apple in February because it hadn’t responded to calls to delete a channel from the National Rifle Association from its streaming-video service after the Parkland, Fla., school shooting.

“Dear @Apple,” Mr. Knight wrote on Twitter. “Your silence is deafening. #BoycottApple.” More than 330 accounts retweeted the message.

How did Mr. Knight post the message? He used an iPhone.

As the reach and influence of Silicon Valley’s tech giants have increased, so have the calls to boycott their products and services. The problem is that pulling off a boycott is not exactly easy: The tech companies’ products are so pervasive that they are difficult to avoid.

That issue was crystallized in recent weeks with Facebook. Hundreds of people deleted their accounts after revelations that the political-data firm Cambridge Analytica had improperly harvested the information of 50 million Facebook users. Yet many of those same people promptly instructed their friends to find them on Instagram, which is owned by — you guessed it — Facebook.