According to a survey by the National Sporting Goods Association, swimming is the 3rd most popular sport based on participation in the US (not necessarily formal participation on a team). An estimated 51,900,000 people age 7 and up participated in swimming during 2010, an increase of 3.4% since 2008. Via Kast-A-Way Swimwear.
The only “sports” ahead of swimming were Exercising with Equipment and Exercise Walking, and well below were sports like Bycycle Riding (39.8 mio), Bowling (39.0 mio), Running/Jogging (35.5 mio), Basketball (26.9 mio), Golf (21.9 mio), Soccer (13.5 mio), Tennis (12.3 mio), Football (tackle) (9.3 mio) and Gymnastics (4.8 mio).
Fastest growing US sport is Yoga (20,2 mio, increase 28.1%) and Aerobic Exercising (38.5 mio, increase 16.3%), while Camping (vacation/overnite) dropped 12% to 44.7 mio, and Boating, Motor/Power 16.2% to 20 mio.
Swimming is also the biggest participation sport in the UK, as we have read for instance several times on www.swimming.org.