It always depends on what console you play on. PS3 may have more than PC. PC may have more than xbox. It's always a matter of when you get on, how good your internet is (For joining games) and what console you play on.
There's about 10 times on X360 as much as there are on PC.
There's a 7K average playerbase everyday on MW2 PC.
Takes about 5 minutes to find a regular domination lobby for me.