"Many social media sites have somewhat been replaced by newer ones in the past few years, after the trend died down."
Social Media is not trend. It is the new platform of delivery for information and entertainment . Years ago TV & radio were 'perceived' as trends when they emerged; but they have existed for years, and generations were shaped & defined by them'. ComScore June 2010 reports that YouTube saw record viewership in May with an all-time high of 14.6 billion videos viewed and that each individual viewer is viewing more videos. http://bit.ly/a8qVnH. YouTube is now 5 years old and nothing has come close to replacing it. In fact, with it's technology and Google, it is likely going to replace TV.