Since its start 20 years ago, online dating has grown into a multibillion-dollar industry that includes not only giants such as Match.com and eHarmony but also niche sites serving older singles, Christians, Jews, animal lovers, vegans and even would-be vampires. As the business adapts to an increasingly mobile culture, more and more people are accessing dating services through smartphone apps, some of which allow users to appraise potential dates instantly and to accept or reject them with the swipe of a phone screen. One in 10 American adults has tried online dating, and nearly 60 percent of Internet users say it is a good way to meet people. Yet some researchers say dating companies' matchmaking algorithms are no better than chance at providing suitable partners. At the same time, critics worry that the abundance of prospective dates available online is undermining relationships. Scammers, meanwhile, are using dating sites to extract money from vulnerable targets, and some dating-site users advise caution about maintaining personal safety.