They are very useful, but they also have disadvantages, for example; you can become dependent of the social websites, they can make an addiction, affects in the human relationships, and always you have the security and privacy risk.
Lately, so many people say that they have a "distance friend", a person that doesn't live in your town, country or continent but you have a relationship with she/he by a social website.
It's very different to have a traditional friend and a friend in Instagram, Facebook... Due to, with a traditional friend you can meet, you know that is real and his/her real reactions in different situations. Instead, with a friend in Instagram, Facebook... you can’t meet, you don’t know if it is real or not and you don’t know how are his/her real reactions in different situation.I guarantee my privacy in social websites by don't telling my password and much information about my private things to anybody, make my account private, and finally, I don’t accept strange people.
If I would make a suggestion to the owners of these websites, I will tell them to create a new option based in to prevent people make the request to be your "friend" or follower, so you click the button and the other people can’t send you the request. Without the necessity to block he/she.