In an interview with GameSpot, Erik Johnson of Valve was reported as saying that the company had no plans to ever sell ad space on Steam because “it would be a bad business decision, let alone just dumb.”
This decision sets Valve apart from Microsoft and Sony who are more than happy to sell ad space on their consoles, despite Microsoft and Sony also charging for online functions.
Valve launches its answer to consoles next month with its Steam Machine PC and Steam Controller. This is likely the reason the question was asked, since Steam making its way into the living room is guaranteed to draw comparisons to competitors like Sony and Microsoft.