Romance is always a top area in mass market sales, but as this article will tell you, those numbers are only expected to increase.
But don't think 'romance' means the old historical bodice rippers or the light fluff that stops shy of heat. According to the same article, Romantic Erotica is a big seller. If you want newer figures (that article is 2000), see these figures from 2004.
For those interested in romance erotica, Passionate Ink.