This is actually great advice! You really should contact her in some way, If you barely see each other anymore the longer you wait the less likely anything will come of it. It's best to just contact her and whatever happens happens, but if you do nothing then you'll never be with her either way. I think an actual written letter would be a lot more personal, impactful and be given more attention and thought by her than just a message over facebook.
Holy shit. If serious have either of you actually been laid?
A physical letter becomes borderline stalking given the OPs situation. The most he can do is send an informal facebook message that says nothing he mentioned in his post other than a mention to sitting next to her once.