These signals (and a number of other factors) are taken into account when classifying if friend requests are spammy / malicious.
(I work on the anti-spam team at Facebook).
The problem is that Facebook needs to both maintain the fiction that it's a network for only your "real life" friends to seem safe and keeping finding new friends to keep the interest up.
But the average person only has about 150 friends and they're either currently on Facebook or they probably never will be. So to get new friends, people have to friend "friends-of-friends", people sharing common interests, people with attractive photos and so-forth. But if Facebook were to really discourage non-real-friends, everyone's friend numbers would drop and the site's excitement level would start going down.
It is a weird kind of situation... Facebook is become more like "the regular Internet" how that works out will be interesting...
Cure to the present roberry 2.0 - gear up a homomorphic scheme[1] combined with a generative personal cloud[2].
The " personal " in PC was most important when C stood for computer. Next, it will be most important when C stands for cloud.
The other wall this epic bubble is going to run against sooner or later - as people wake up at an intuitive level :
"Well, since Moore's law makes computation really cheap, let's just give away the computation, but keep the data."[3]
All of this nightmare will be compounded by the stunning crap-storm about to emerge in economies world over.
[1] - http://crypto.stanford.edu/craig/
It's not perfect but with a little bit of organisation you can use the same FB account for random people, casual acquaintances and real friends without causing issues.
Hopefully this will only get better.
As long as the profile is a young attractive lady, that option will rarely be chosen.