The Maine Historical Society writes "Postcard valentines were fashionable in early America, often collected and pasted into albums. More elaborate valentines became popular in the mid-1800s. This exhibit explores the evolution of valentines. Perhaps you will be inspired to make your own!"
So hang on to those Valentines you've received; maybe some day they'll emerge from a dusty box, inspiring a future amorist to take heart (pardon the pun).