It may well have come up in your household because Pinterest.com – the photo-focused social network service that started in 2008 – seems to have gone viral all of a sudden. Unique visitors to Pinterest.com grew 400% last fall, CNN reports, calling it “2012′s hottest Web site.” Why so viral now? FastCompany suggests it’s 1) the exponential-growth factor (what I pin gets “re-pinned” by my followers, whose stuff gets re-pinned by their followers with a Twitter-like re-tweeting snowball effect), plus 2) Pinterest having figured out better than anyone how to host people’s passion for collecting stuff. If you’re wondering what the site is like, well, the name suggests pins on a bulletin board – the physical kind, not the electronic kind of pre- and early Web 1.0 days. It’s like a social scrapbook, because FastCompany has one expert describing it as a tool for creating “collages of personal happiness.” Also kind of the anti-Facebook, as in “a refuge from relationship status, check-ins at restaurants, or pictures of kids.” These analysts make it sound like a cross between Twitter (brief bursts of public posting), Facebook, and an escape from them both – but still social media. It has also been called “a reddit for women,” referring to the social news site reddit.com. Unlike at Facebook and Twitter, “97% of Pinterest’s Facebook fans are women,” Geekosystem reports, citing AppData. To say we’re seeing yet more social-media market segmentation is to state the obvious. The site is also reportedly quite addictive. People had New Year’s resolutions about cutting back on Pinterest, FastCompany reports.