If your marketing plan assumes that only teenagers are adopting social media, you could be making a very costly mistake. Nearly half of Americans between 70 and 75 are now online, according to the Pew Internet and American Life Project. “Social media are giving seniors a voice, and for a generation that has sometimes felt isolated and overlooked, that’s no small feat,” said Sarah Hoit, chief executive of My Way Village, which designs online social networks for older adults. For more social media news, check out SmartBrief on Social Media.