I don't have or need a lot of friends. The ones I have, are dependable and can be trusted without limits. There others, more like people I associate (tolerate) with, but wouldn't lend money or trust with my car or house.
Some people need a lot of friends, and I find that many of these are are operators and manipulators. They treat other people like resources, and it's kind of sick seeing one of these types trying push their way into your life.
Friendship take time to grow, it either happens or it don't. I've never asked, or really declared a friendship, it's just something you know when it's there.
When some stranger, or new user, just out of the blue requests that you be their friend, it really puts most people on the spot. Most of us, are decent enough not to want to cause offense (well, most of the time...), but a friendship is an important and valued thing, not to be passed out like business cards.
Perhaps, the new users are just testing out the site options, and not really sure what to expect. When someone asks me if I'm their 'friend', I tend to expect to be hit for providing a service...