MS may be third as of late but it's absolutely useless for them to fight for marketshare right now. Their focus should be to recoup as much as they can from the Xbox brand investments (and to make up for their financial errors ie. RRoD and e74, unprofitable software, moneyhatted exclusives that lost them money, etc.) The third-party publishers are firmly behind both companies. Just look down the PS3 exclusive list. What do you see? First-party Sony games, Japanese games (because PS fanboys whined about FF13 going multi-plat so S-E gotta give them Versus. And a number of Japanese devs favor the PS3 or don`t have faith in the 360 at all. Not much MS can do about that. 360 is weak in Japan so their best bet is to partner with independent Japanese deveoplers and secure new Japanese IPs for the Microsoft Game Studios label and build on the Japanese IPs they already own) and a couple Rockstar games that were planned as PS3 exclusives since near the beginning of the gen (because Rockstar wants to see what they can do with a open-world sandbox using Blu-Ray storage capacity. It has nothing to do with market share at all. Rockstar was always tight with Sony and they want to experiment with the console. Just like how Sega, EA, Ubisoft, etc. likes to experiment with Wii exclusives.)
Ditto for Sony too (re: marketshare), they are fine now, they just gotta work towards making some money now.
I think MS and Sony are more concerned about Nintendo. Hence NATAL and Arc. But they have an uphill battle on their hands at this point.