I agree with RebeccaWithTheGoodHair "I don't think you should leave but YANBU to find it upsetting."
I think your friend who told you about her Facebook status is either kind and felt you should know or bitchy and was string. Which do you think?
Either way, make it clear to your Facebook-reporting friend you do not want to know what this woman says or does on Facebook unless she names you in person in which case you only want to know if your Facebook-reporting friend will provide you with a screen shot so you can take it into work and see if this will convince them to speak to this other woman.
"She's been there 15 years, she's a lot older than me she's in her 50's and I'm in my 20's which is why I find her quite intimidating." Please start to read up on asseriveness and do not allow this woman to intimidate you at all.
If the job affects your mental health, by all means leave. Otherwise force her to be professional with you by being nothing but professional with her. Be the better bigger person and she will either have to shape up or ship out! She is creating this conflict.
No more chats about burring the hatchet. Make a point of inviting her to everything you would invite others to, in front of others, she can choose to come or not but all your colleagues will know you invited her.
Do not engage in any back biting or in office gossip and do not be passive aggressive with her. Give her no ammunition to fuel her dislike.
"I just find it so hard when someone asks me how my weekend was and I can clearly see her pulling faces when I talk I find myself just saying "fine" now rather than going into detail"
You know if she is pulling faces she is just making herself look silly.
I agree with RatherBeRiding "if you catch her making a face when you say something, stop, back up and call her on it (but nicely and politely!) "Is there something you didn't like/something you disagree with/something you find amusing about that remark, Bitch Colleague?" with a big smile on your face. She will look a twat, other colleagues will see her looking a twat, and you will have the moral high ground and the satisfaction of knowing that SHE knows you're not bothered by her bitchiness."
Then, move your chair so you cannot see her and speak about what you want to speak about. If you catch her making face just make a note of it. If it happens a few times go into HR and ask if you can be moved out of earshot, you can say you find her behaviour towards you unpleasant. They may not believe you but I think they should investigate. Can others see the face pulling.
She sounds utterly pathetic, why are you allowing her to silence you. You do not need to allow her to do this.
"Her husband and children are extremely pleasant and will often say hello if I see them dropping her off or picking her up. I just don't understand what her issue is, it's wearing me down"
Maybe her husband made a compliment about you? Weedgarden why does she not like you, who knows! Maybe she wanted the job you got or maybe you are younger, or slimmer, or posher, or better dressed than her, maybe you remind her a school bully or a woman a previous partner heated with. Who knows. Who cares.
Do not be rude or bolshy or passive aggressive, these things seek into your life too. Be nice, normal and rational.
If she is undermining you in the office it is an HR problem, compile your evidence/record of incidents, especially anything relating to work, where she is difficult and makes it hard for you to do your job, and be ready to talk to HR.