The company claimed that the adverts are inserted without affecting the nature of the game itself, and do not affect functionality or bandwidth.
Massive has been running adverts from 30 different companies since it launched its network in October last year.
The adverts typically appear as 15-second pieces of audio or video when players enter selected areas or zones within the game.
Nate Elliot, an analyst at Jupiter Research, said: "This is an interesting development but it has to be pointed out that these are not console games, which is where the real action is. I think they will want to move into consoles if they can make a success of this.
"It is a sign of the growing desire of advertisers to reach a young male market that is increasingly shunning mainstream forms of advertising.
"Console players have a history of buying technology, so they are attractive to [advertisers]. But the main trend so far has been for advertisers to sponsor a game so that their products are integrated into the game itself."
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