That's because nobody actually has 276 friends. Most people probably don't know you that well, or even think of you that often. Not being rude, it's like that for almost everyone. And the ones who DO think of you possibly don't go on facebook all the time. I have 65 facebook friends - all real life people I know and have mostly met... There are 3 friends I met through farmville circa 2012-13, who I never met (live 500 to 1000 miles away,) and 3 penpals from the USA and Canada.
The other 59 are close family, friends of adult DC, extended family, colleagues, DH's colleagues, half a dozen neighbours, 7 or 8 friends. I got 27 birthday greetings earlier in the year So approx 40% of my 'friends' on there wished me happy birthday, and that's because I know them and have regular contact with many. The other 36 people who didn't, as pps have said, don't go on there all the time, and I don't have contact with them several times a week. I don't go on it for 3-4 days at a time sometimes, and miss birthdays myself sometimes too.
I know several people who have 300 'friends' on there who get less than 30 happy birthday greetings. Really depends on how often you are in contact with them day to day, and week to week, and how many of them use facebook often.