To be clear, you don’t need to use hashtags to conduct a filtering search on Twitter. And if millions of people are tweeting about a new Star Wars trailer, all at the same time, the odds that your tweet will surface and find a huge audience are minuscule.

But Robert Hernandez, a digital journalism professor at the University of Southern California, said “there is still something wonderful” about watching a community come together in real time “only because of a hashtag.”

Mr. Hernandez pointed to #yesallwomen, which allowed hundreds of thousands of strangers to discuss violence against women and reveal their anger and horror about abuse and sexism. Mr. Messina cited #blacklivesmatter as an example of a hashtag that is “powerful and necessary.”

He doesn’t think badly of people who use #ROFL to denote “rolling on the floor laughing.” That person is “trying to express something,” he said, and you have to take the good with the bad.

Mr. Hernandez is also a fan of hashtags; he’s a co-founder of a weekly Twitter forum about online journalism called #wjchat. But he acknowledged that they can grate and irritate.

“Like anything that becomes popular — especially if it was organic and grass roots — corporations take it over and incorporate it into marketing,” Mr. Hernandez said. People are unlikely to use hashtags created by brands, he added, no matter how much they are promoted.