It's hard not to take a facebook friend rejection personally, but the reality is that it just facebook and there may be a number of possibilities going on. It is clear that this girl is very selective about who she adds.
Just because she may recognize you from work does not mean she feels close enough to add you, as lately many people only add people that they are actually friends with rather than, say, someone from work that they only occasionally talk to. It doesn't mean she doesn't like you, you are just beyond the limits of who she deems suitable as a facebook 'friend'.
The good news is that you can do something about it...just start interacting with them more. If you would like to get into this inner circle, do it very gradually, and perhaps start with one person at a time. Perhaps if you are both on a break, you can approach this girl or someone else in the office and perhaps do a bit of small talk...even if it's for 30 seconds. A little bit goes a long way. As long as both of you aren't interfering with your work, go for it.
I would slightly agree with the other posters in that a social life and work are best kept separate...but not everyone wants that. Some people clearly see that they are left out and would like to be a part of things, like the OP.
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Given a “tentative” diagnosis as a child as I needed services at school for what was later correctly discovered to be a major anxiety disorder.
This misdiagnosis caused me significant stress, which lessened upon finding out the truth about myself from my current and past long-term therapists - that I am an anxious and highly sensitive person but do not have an autism spectrum disorder.
My diagnoses - social anxiety disorder and obsessive-compulsive disorder.
I’m no longer involved with the ASD world.