Facebook - Personal and business. A ubiquitous platform for people to keep in touch with old and new friends, acquaintances, co-workers, classmates, and family, share information and media, and to engage with "fan pages" (usually a business or organization) or communities based on personal interests.
Twitter - For business mostly. It seems that people sign up for twitter to follow their favorite businesses and organizations, TV shows, news media outlets, public figures and celebrities, random people that tweet about nonsense, etc. As you can probably tell, I don't use this platform that often nor do I care for it.

YouTube - Personal and business. For entertainment or educational purposes, I follow about a dozen YouTube Vloggers (a.k.a. YouTubers) who have anywhere from 20,000 to 5,000,000 subscribers on their channels. After reaching a certain number of subscribers, these vloggers sign-up and get approval to become "YouTube Partners" where they are paid by YouTube, through advertisements. Some of these YouTubers make a decent living from posting videos and have no need for a regular 9 to 5 job, and some have e-commerce websites selling goods and/or services and post videos on YouTube to build a fan base, and drive that traffic to their websites and/or to their affiliate or sponsor links.
Vine - Personal. Vine users can post creative mini video clips and share with others. Haven't seen any businesses utilizing Vine...yet.
LinkedIn - Business. A platform where people can network with other professionals in a business setting, and post and read articles related to their businesses and industries.