NEWSFLASH: Not everyone has friends online every time they want to play a team game. Obviously they want a casual game that 4v4 can offer. What they don't want is an incompetent person that has no business operating a computer joining their team. The game can be fun playing with random people of lower skill. It is not fun when the person playing with a hammer is on your team - the type of people this discussion is about.
For a constructive post then:
The way I have tried expanding my friendslist for 2v2s (or bigger) is the following:
When I play with ppl both with, and against, that I seem to connect with, I add them to steam. Either they play well, provide intelligent resitance and or are nice in chat.
Once friended, I usually try to get a game with them quite early so they dont forget who I am and what the heck Im doing in their friendslist. Not all of them might be players I play regularly with, some I havent played with still. But the list grows all the time. And its fun to get to know new ppl.
Tip of the day: Be social and add the ones you connect with, and you will most likely end up with a bigger friendlist.