Let me explain.
-- You log on to Internet.
-- The information requesting the log on is sent to the Internet, est. 5 kb used of your 100Mb
-- You are now logged on, the internet sends you the landing page, est. 10 kb used of your 100Mb-5kb
-- You now click to enter Facebook, and that information to request facebook is sent to the internet (more data transfer)
-- The facebook home page is sent to your cellphone (more data transfer)
-- You now send in a command to look for a specific person (more data transfer)
-- The internet sends the search results to your phone (more data transfer)
-- You click the person you are looking for, sending a request to the Internet to download that person's page (more data transfer)
-- The page is opened and sent to your cellphone (more data transfer)
-- etc. etc. etc.
Now, cellphones have small amounts of memory, not like a PC. And, using FaceBook is a far cry from downloading music, which can be 5mb or more, which would eat up your 100mb quickly!
But, using FaceBook, my bet is that you will have more data with 100mb than you can use in a month, provided you avoid gawking at high-resolution photos or watching videos (which are very large!) So, I think that you will be able to do a lot!
Most companies, and I imagine yours as well, will provide a way for you to track your use.
Most of these, though, are listed in Kb or (God forbid) in bytes, so here is some more information you may need:
1 Mb = 1,024 Kb
1 Kb = 1,024 bytes.
So, if it's listed in bytes, to determine your Mb use, divide that result by 1,024 then divide by 1,024 again.
If it's listed in Kb, to find your Mb, divide by 1,024.
If it's listed in Mb (hopefully so), you don't need to do any math.
Probably a lot.
The best suggestion I can give is for you to try, and track your use to make sure you don't go over.