Idle dwarves will hang out in meeting areas and talk to each other, thereby making friends.
Now, if you don't mind tantrum spirals, or if you're REALLY good at keeping the stupid little bastards alive, this isn't a problem. If you don't want a tantrum spiral the first time a dwarf dies, though, you're going to want to try to keep them as busy as possible so they don't have time to make friends. Personally, I accomplish this with having them haul stuff. In my current fort this means hauling green glass goblets (my primary export) up from the magma glass forges, food, blocks, and collecting sand, mostly.
As Leonidas said, your hauling needs will tend to come in waves. When I hit a low point, I just set some loose stones to be dumped to keep them busy. I also periodically have my most recent wave of immigrants all head to the drowning chamber.
Edit: Regarding dwarves making friends. Normally this is a problem because a dwarves will spend enough time talking to make friends, but not enough time to get thier social skills up very high. Consequently, when one of a dwarf's friends dies, his other friends won't have the skills needed to improve his mood enough to prevent a tantrum. So, basically you have two options:
1: Make sure your dwarves have NO downtime (aside from eating/sleeping/drinking/breaks)
2: Give your dwarves LOTS of downtime (i.e. everybody gets to take winter off every year, or something. You'll have to make sure each dwarf periodically gets some extended downtime to keep thier social skills from rusting into uselessness.)