Things have gone pretty well for Wayfair Inc. lately. Revenues surged to nearly $1 billion last year. Its sales jumped about 50 percent in the first six months of the year from the same period in 2013. It just launched a successful initial public offering, raising $ 319 million. But Boston's biggest Internet retailer has a big problem. Nearly half of its target customers don't know it exists. Brand awareness has proven a particular challenge for Wayfair, which sells home decor and furniture over the Internet. A survey in August found that only 52 percent of middle-class women between the ages of 35 and 65, Wayfair's best shopper, were familiar with the company.