Thank you notes for birthdays address the recipient and make a specific reference to anything that she gifts to the writer. Every note should bear a unique personalization, even if the recipient only sends a card, attends the party or brings refreshments to the party rather than a present. If the writer gives his thanks in person, he isn't obligated to write a note, though writing one remains a thoughtful gesture.
About.com supplies several examples of thank you notes in the website's etiquette section. Notes, unlike full letters, only need simple salutations and closings. The formality of the thank you note depends on the writer's relationship with the recipient. Brief mentions of personal uses or anecdotes about the gift are appropriate in notes to friends or family, but something more succinct works better for acquaintances and co-workers. The writer should confirm who has given him a particular gift before writing a thank you note to that person. A list of names and the corresponding gifts works as a useful reminder if it's written as he opens each present. A handwritten note with appropriate stationery works as well as an e-mail or a Facebook message if it's heartfelt and well composed.