There are people who stand for social networking sites while others are against. There are advantages for users but also disadvantages.
I use some social networking platforms and I´m gonna talk about the advantages of social networking sites:
1 - Some social networks are good opportunities to find a job - Nowadays the big companies have specialists on human resources that search for profiles in these kind of sites to recruit new collaborators. If you´re looking for job you can create a profile (a serious, professional and complete profile) and you might be discovered and recruited by a company. You can describe your capabilities and publish some of your works - and with a little luck you can find a job.
2 - You can meet new people - social networking sites are the perfect spot to neet new people online. With the help of advanced search tools you can find users who have the same interests you do and that can be the starting point of a great friendship and/or relationship. Many people meet online and then stabilish a relationship in real life - getting married, having children, etc.
3 - You can overcome your timidity - Contacting with other people through the internet becomes more easier for shy people. Shy people can overcome this using social networking platforms to stabilish contacts. Later the interaction it´s more easier because they have already talked and interacted online.
4 - You can promote your site or business for free - If you have a business or site you need to promote you can do it by creating a business profile on some of these networks. You can load images, your logo, etc. into your profile and it will be visible for your virtual friends and all the users who visit your profile. You can also put topics on groups related to your business and stabilish new contacts that can lead to sales, subscriptions or anything else.