My son is currently in BC and was there for Christmas.  This suggestion, though, can be used anytime.  I posted on my wall, my husband's wall, and my son's wall that I would mail JT anything that they posted in response to the comment.  Once it was done, I printed the posts out and mailed it to him.  A large part of the posts were people he didn't even know (mainly my husband's and my friends) who thanked him for his service, many former sailors who gave him advice, etc.  It was very moving and he really appreciated it.  Here's what I posted to our wall:

THIS IS YOUR OPPORTUNITY TO SEND A MESSAGE TO "Recruit's Name" IN BOOT CAMP!!!!! (Even if you don't know "recruit", just saying "Thank you" is appreciated more than you could imagine.)
"Recruit"'s mom here. As most of you know, "Recruit" is currently in Navy boot camp. Reply to this post and I'll mail it to him so he knows you're pulling for him. (Be encouraging, he's going to need it.) It's currently 14 degrees at Great Lakes RTC and he's probably running on 5 hours of sleep a day, so anything you can post will be appreciated (memories, jokes, photos, whatever.) I will print out and snail mail every response to this post on Saturday.

A word of advice, my son had his FB page set up to approve all posts to his wall, so it didn't show up when I originally did it.  You may want to check their wall to see if it's there after you post.  

I eventually went in and approved it on his phone so that it would show up.  His FB account was password locked, but the browser on his phone still had him logged in and I approved the post for him there.  I only mention this because it increased the amount of responses I got.  He later sent his PW for me to post his address (which I'd already done) and we don't really have privacy issues, but please consider only doing this if you don't think your recruit will feel it's a violation of trust.  I do NOT advocate breaching their privacy like that if you are not 100% sure they are okay with that.