Facebook Inc's No. 2 executive, Sheryl Sandberg, paid a Mother's Day tribute to single moms, acknowledging she never fully understood the difficulty of raising children without a spouse or partner until her husband died last year. "I did not really get how hard it is to succeed at work when you are overwhelmed at home," she wrote in a weekend message posted to her own Facebook account, adding that the death of her husband, Dave Goldberg, "has redefined what it is to be a mother." Sandberg, 46, who joined Facebook in 2008 as the chief operating officer for the popular social media platform, lost her husband to a treadmill accident while they were vacationing in Mexico in May 2015. Then, without warning, I was on my own," Sandberg wrote.