Try this:
First add a column (let's say F) that returns the month of the date (D)
F1=MONTH(D1) (and drag it down)
Then, count:
=COUNTIFS(F:F;4;D:D;'Yelp');
That means: count only when month is 4 and source is Yelp
EDIT:
I assume you'd want to filter by year as well. In that case, add a column that returns the year:
G1=YEAR(D1) (and drag it down), and then:
=COUNTIFS(F:F;4;G:G;2018;D:D;'Yelp');
That means: count only when month is 4, year is 2018 and source is Yelp
EXPLANATION:
Syntax of COUNTIFS is:
COUNTIFS(criteria_range1, criteria1, [criteria_range2, criteria2]…)
The COUNTIFS function syntax has the following arguments:
criteria_range1 Required. The first range in which to evaluate the
associated criteria.
criteria1 Required. The criteria in the form of a number,
expression, cell reference, or text that define which cells will be
counted. For example, criteria can be expressed as 32, ">32", B4,
"apples", or "32".
criteria_range2, criteria2, ... Optional. Additional ranges and
their associated criteria. Up to 127 range/criteria pairs are allowed.
From: https://support.office.com/en-us/article/countifs-function-dda3dc6e-f74e-4aee-88bc-aa8c2a866842
