This involves finding three "social" events, which are a white whale, a royal convoy (NOT a naval convoy), and a social chest. When you pass by one, it should show up blue on your map and should automatically share when you get close enough. You HAVE to be online and connected to Uplay for these events to appear. These events also have a limited amount of time for you to find them before they disappear. There is a map of them here
(I do NOT take credit for this map, found it randomly while searching online for help.)
When the game first came out, social events were everywhere. Ubisoft severely nerfed how often they turn up, so much so that I have 99% completion and have not seen a social item. It is unconfirmed whether or not adding more friends that play ACIV or adding more friends on Uplay affects how often they turn up (although I've heard more that it's your Uplay friends that affect it) so my advice would be to add friends, and when they find a social item go to where they found it and see if it's there for you. The gamefaqs boards for this game has a lot of people adding each other and posting their coordinates. If not just use the map and hope you get lucky. Ubisoft has realized that they made the events too rare and are planning a fix, but there is no date scheduled yet of when that will be, so if sailing around is getting boring I would also suggest just waiting for the update. I will update this solution as I learn more.
EDIT: I just found my first white whale, shortly after accepting a friend request on Xbox. Apparently just add as many people as you can and if they find a social event it'll show up on your map.
EDIT: Found a royal convoy! What I did was went to acinitiates.com and looked at the xbox 360 sighting threads. Find one that was posted recently and go to the coordinates. If it's not there, either reset your xbox or put in the multiplayer disc and start up multiplayer (apparently these actions "reset" the server for finding social events) then head back to the coordinates after you start the single player back up and they might be there. Repeat until necessary. It's a long shot but it worked for me!
EDIT ONCE MORE: I found the last item I needed, the social chest. (Credit to Toxin066 for finding this checklist, credit to LDL from ps3trophies. org for making the checklist.) https://docs.google.com/spreadsheet/ccc?key=0AvhhVsGK1kB-dHp...
This spreadsheet shows you pictures and possible locations for every chest, convoy, whale in the game. Resetting my xbox worked for finding a naval convoy, but not for the social chest, so I just went to every possible location where it could spawn and eventually found the chest in Grand Cayman on the dock. It'll probably be gone by the time you read this, but you can just visit all the possible locations and hope you get lucky.
EDIT: Credit goes to TheBeast0997 for finding this
"x AL3X Zz x
"A tip that may help some people; I haven't found a single thing prior to this white whale, even though I've been visiting the coordinations given by many over the last week. Saw the Illustrious Pirate Pack available through the season pass (it'll cost you if you haven't got sp) and grabbed it. It took me almost no time before finding my first social event when loading the game back up." So apparently if you download the Illustrious Pirate Pack you might have some more luck finding these events.
Since some people are complaining I'll add this disclaimer: there is NO GUARANTEE you will find ANY social event by using these maps or other methods. This achievement, until they patch it, is pretty much 85% luck.
EDIT: Someone else found a solution to some of the social events not being there even if they're on the map. Credit to this goes to StockierHeart.
"If you can't find anything now, and your friends have shared something that should still be there here's the solution !!
When you launch your game and press the "Start" button it says: "logging in to ubisoft servers" now what you gotta do is press (B) to cancel the conection, it will now ask you if you are sure about it, choose "YES".
Once you enter the game go your Jackdaw and enter the Kenway's Fleet... NOW LET IT CONNECT, DON'T CANCEL... once it's connected you can close the Kenway's Fleet and the share dicoveries will pop up on your screen :) :) :) :)
Edit: Notes: I have 98 Xbox Live Friends
I loaded the game normally first, then I gone back to the main menu and launched Uplay... waited for it to loaded and the exited. At the menu again I pressed "Start" and canceled connection with ubisoft servers.
Once I loaded the game the Wi-fi icon near the map was off (a friend of mine is having trouble on this, he cancel the conection but when he loads the game it's alredy connected), then I entered Kenway's Fleet and speeded up all remaining friend's ship and once I closed the game the discoveries popped up here.
This is only for people who can't get shares from friends, not for those who can't find anything on their own but still getting shares from friends.
It doesn't work for everyone, sad."
FINAL EDIT I PROMISE: It appears as of January 14th that Ubisoft has fixed the social events, as everyone has been finding them quite easily now. So get out there and start hunting!