In her original post Rebecca shared lovely details about Hallee's personality and mentioned that she was autistic, before asking whether anyone who saw the post if they would mind sending cards to her little cousin this year, so she wouldn't have to feel like she was celebrating alone again.
Rebecca's plea obviously pulled at a few heartstrings and thousands of lovely people from all over the world have since responded. In fact, according to stuff.nz all of Hallee's cards are having to be transported via trucks with more than 10, 000 letters and parcels already having arrived at her door - overwhelming her local post office.
Gifts and messages have included well-wishes from soldiers in Afghanistan, greetings from the whole state of Ohio and a space Barbie doll from NASA.
Hallee's family have thanked everyone who have sent birthday wishes, saying, 'Our gratitude is hard to put into words,'
'This is all reaffirming that most people are kind and loving, and that we all have the capacity for caring about others, even if they're many miles away.
'We're all extended family.'
To keep all their well wishers up to date Hallee's family have set up a Facebook page called Hallee's 19th birthday where they've been uploading images of all the cards and gifts the teenager has been receiving in the run up to her 19th birthday on the 2nd of July.
The story all started when Rebecca, from Oxford, Massachusetts, uploaded an image of her cousin Hallee sat eating her birthday cake alone, after none of her friends or classmates turned up to celebrate with her.
'Everyone please read: I need a favour', Rebecca wrote. 'This is my cousin Hallee. I'd like to tell you a little bit about her...'
'First off, she is a beautiful young woman-both inside and out. She lights up a room with her smile.Hallee is funny, sweet, caring, smart, an athlete, a jigsaw puzzle champion, a wonderful student, and a best friend to all. Hallee is an amazing person-a person I am proud to be related to. She is also a person who just happens to have autism.'
She adds that Hallee has never let her autism define her, but she's still experienced some tough times, including, as Rebecca explains, a lack of guests at her 18th birthday party.
'She had been so excited to have a party with her friends. She wanted to go bowling, have fun, and eat cake and ice cream. Invitations went out to her classmates at school and to other friends in the area. Hallee sat at her party anxiously waiting for her friends to arrive so they could have fun....but Hallee's friends would never arrive.'
'Not a single one. Below is the picture of her celebrating alone - eating her birthday cake. My cousin is a beautiful young woman who will always have the mind of a child...so as you can imagine, she was heartbroken and beyond sad. She was hurt...'
The emotional tale, however, has a happy ending, as Rebecca ended her message with a plea to send Hallee birthday cards 'to make this year's birthday incredible!'
'I would love to flood her mailbox with birthday cards, from all over! Hal loves getting mail - this would be the best birthday gift she could ask for,' she said. 'If you could find it in your heart to take a few mins out of your day and send her a card, I would be forever grateful.'
'This would mean a lot to her mother as well, for as you can imagine, watching your child cry into her birthday cake breaks your heart. If there is anyone who deserves a great birthday, this is the girl.'
Since the post went up on 8th June, it has been shared more than 122,000 times, with countless commenters assuring Rebecca that Hallee will be having a much happier birthday this year. She's even had to decline the kind offer of presents, saying 'This is incredibly sweet of all of you and my family is very appreciative. That being said, gifts truly aren't necessary. A nice card would be just fine!!!'
If you'd like to send Hallee a card for her birthday, which is in July, you can post it to: Hallee Sorenson, 34 Wellesley Way, Bangor, ME 04401.