There is a big enough player base. But maybe not in all regions and not in all game modes at all times of the day. The problem is that the player base is split between ranked, unranked, quick match, brawl, …
I usually play QM, but tried out Unranked couple of days ago. I played on Europe server about 5 games in a row and every game there were at least 2 players on my or enemy team I have encountered in the games before. Que time was also at least 5 min per game.