An insincere and self-promoting post on another's Facebook wall intended for a public audience, in spite of the wall's one-on-one nature.
This is usually done to advertise traits which when put in the direct form of a status update would make the poster's motives far too transparent.
KATE: Why did Christina post "you're the best. LOVE LOVE LOVE." on my facebook wall? She hasn't talked to me in five months and it's not my birthday.
MARK: She probably wants everyone to think she's the type of girl who is tight with her friends, and people would call her on it if it were a status update.
KATE: A proxy wallpost? Seriously?
A post intended for a large audience yet posted on a single person's wall to appear more genuine. Since proxy wallposts are visible through homepage feeds, one can advertise a trait of oneself that would be construed as bragging if in the form of a status.
Suppose you want to advertise to your friends that you are caring and social. This is impossible to brag about, so instead just find your friend who shares the most mutual friends and write this gem of a proxy wallpost:
"We had such a great time last week! I miss you so much <3"
Soon, everyone you know will see it in their feed. And, unlike a status, no one will call you on it for fear of you playing the None Of Your Business card.