What are the most popular songs in the land? As the music business has changed, it’s been harder and harder to tell. Some songs have made the bestseller list for sales of only 5,000 copies. “This week, Billboard began figuring in download sales in its main pop singles list, mixed with the two other factors it long leaned on — radio play and retail sales. At the same time, the magazine has introduced a new, download-enhanced chart that more accurately measures those songs that truly are the most popular.”