This might seem exciting, as though you’ve won a lottery. But in reality, when you recheck your lottery ticket, you might find you were off by a single number.You see, being in the Billion-Dollar Start-Up Club limits how, and if, a company can get out of the Billion-Dollar Start-Up Club — at least safely.The Club is growing quickly. Based on recent financing rounds and stories about the companies, Twitter is valued at $8.5 billion; LivingSocial at $5 billion; Dropbox, $4 billion; Square, $3.25 billion; Spotify, $3 billion; Rovio, $3 billion; Airbnb, $2.5 billion; Pinterest, $1.5 billion; Box, $1.2 billion; Gilt Groupe, $1.1 billion; and Evernote,