Despite the social networking flooding Internet, television and billboards with its hashtags inviting people to log on, four out of five Americans with Internet connections still don't use it regularly.
One problem might be generational, experts say; for younger people who grew up texting, Twitter's limit of 140 characters in tweets isn't a problem, but for older people, the constraint conflicts with their "natural mode of communication," Dhiraj Murthy, a professor of sociology at the University of London, told The Wall Street Journal.
And Twitter relationships are different than those on competitors like Facebook and LinkedIn, he said; Twitter connections tend to be acquaintances, humorous strangers and celebrities.
"You obviously may not know everyone on Facebook very well, but overall you tend to have stronger relationships there than with the people you follow on Twitter," Murthy said.
While Twitters is useful to journalists, public figures, activists, entrepreneurs and businesses, there is less appeal to the average Internet user, MIT psychology professor Sherry Turkle says.
"The mandate is different on Twitter -- you have to be interesting," she says. "You need to develop a voice, which is something Facebook doesn't ask of you."
Twitter still struggles to keep up with Facebook, which has more than half the U.S. online population connected and more than five times the world-wide users of Twitter.
Early in 2013, Twitter executives had projected 400 million monthly active users by year-end. Instead, Twitter reported 241 million active users, adding just 1 million in the United States and 8 million abroad in the fourth quarter, the Journal reported.