It seems that nearly 30 percent of young people 18 to 34 who send messages and pictures on social media find themselves regretting it. So suggests a poll by what is called a legal information website FindLaw.com. The experience seems to mirror the same regret long-identified among e-mail users. The warning that “E-mail is forever” seems to apply. It’s a nice little irony that so much digital communication is erratic but that witless and embarrassing messages and pictures are there for the world.