Month on Month - % difference

Give this a try:

First you will need a date table, there are a few threads around if you don't already have one.

Next, if you don't already, create a date field in your Facebook data that aligns all of your dates to the First Date of each month.  This makes Date math later a lot easier.  A simple calculated column like this will work:

WorkingDate = DATE(
    YEAR('YourTable'[DateField]),
    MONTH('YourTable'[DateField]),
    1
)

Create a measure that calculates the overall metric you are trying to achieve.  This might be something like

Post Count = CountRows('YourTable')

Next we want a measure that is going to do the Month By Month restricting:

Post Count Delta = CALCULATE(
     [Post Count],
     DATESINPERIOD(
            'DateTable'[Date],
            LASTDATE('DateTable'[Date]),
            -1,
            MONTH
      )
) - CALCULATE(
     [Post Count],
     DATESINPERIOD(
            'DateTable'[Date],
            EDATE(LASTDATE('DateTable'[Date]), 0),  // This is the prior month
            -1,
            MONTH
      )
) 

Hopefully this gives you an idea to try.