In one sense, nothing at all has happened to US two-dollar bills. They are still in circulation and are still considered legal tender. The fact remains, however, that relatively few people would be able to produce bills of this denomination from their wallets on demand these days. Even at the height of their popularity, during the 1950s and 1960s, two-dollar bills were rarely given out as change or stored in designated cash register slots. If it hadn't been for a renewed interest during the country's bicentennial in 1976, the denomination may have been completely phased out.
Many people believe that two-dollar bills are so rare or so collectible that hoarding them makes more financial sense than spending them. The truth is that most of those in circulation today are worth exactly $2 US Dollars (USD). They are not especially rare, at least not from a coin collector's perspective. Federal reserve banks still order them to replace ones pulled from circulation due to condition or age. The reason many of these bills are not seen on the street is that recipients tend to save them as curiosities or collectibles rather than put them into general circulation.
There are organizations and individuals who actively promote the use of two-dollar bills as everyday currency. Many bills are marked "this is not a rare bill" to encourage others to spend them like any other denomination. The gift shop at Monticello, the homestead of Thomas Jefferson, is said to routinely give them out as change to honor the president featured on the face of the currency. There are rumors of certain store owners not accepting this denomination from customers, believing that the bills are either counterfeit or no longer considered legal tender.
Two-dollar bills may suffer from the same perception problems as the Susan B. Anthony silver dollar coin or the recent Sacajawea golden dollar coin. Few vending machines are set to accept them, although they are generally accepted at self-service grocery store checkout stands. These bills seem to be most popular as tips, although there are rumors that certain military members and out-of-state visitors will deliberately spend them to prove their impact on the local economy.