I'd argue that the opposite is the case - MMOs have built in friend tracking and communication (along with VOIP obviously) so id be willing to be only a minority of MMO players use services like x-fire.
Let me explain a bit about data and statistics.
The very fact that MMO players do use x-fire, albeit lets assume they are in the minority, the data is still absolutely valid. That is to say it is factual information. The fact that the percentage stats for MMO players is pretty consistent from day to day, shows that enough data is being gathered to make extrapolation predictions.
For example in looking at the link
http://beta.xfire.com/games/swtor
we see that it is based on 2624 players (at time of looking). Now as pointed out by Xylak, there is 1.3 Million subscribers to the game, which means that pathetic 2624 number of SWTOR players is only 0.2% of the total amount who play the game, so yes I agree it does looks like 99.8% of people who play SWTOR don't use xfire, but the 0.2% that do provide useful factual information that can be extrapolated. As I said The fact that the numbers are fairly consistent prove this - I check them everyday believe me they are!
Back to my election comparison there is some 30million people who can vote in the UK, with a pole of 1000 they can predict quite well the result of an election, that is 0.003% a lot less that the 0.2% we have with SWTOR.
xfire stats are far more accurate than you may think my friend.
I am happy to give more explanation if required as there is another point I could make, but am very aware I am ranting on a bit.