Social networks have their origins in 1995 with the creation of classmates.com, allowing people to reconnect with friends from school. In 2002, sites promoting circles of friends online popularized the term for virtual communities. Facebook was created for universities in 2004 but has since expanded to all internet users. Social networks have advantages like facilitating work and education collaboration. However, disadvantages include privacy risks from hackers and national security threats in some countries. Additionally, overuse can become addictive. Popular social networks include Facebook, the largest with over 900 million users; Twitter for microblogging with photos and hashtags; Google+, a Facebook competitor; and Yahoo Answers, an alternative to Wikipedia for asking and answering questions.