First, you would clarify if this question is primarily about root-cause analysis, i.e. identifying the reason for the 15% drop.
Assumption: Facebook Groups are defined as a group of Facebook members that share a common interest, organization, family, etc.
Questions to ask to root-cause the problem:
a. Is the 15% drop YoY or over the projected growth?
b. Is the 15% drop for a specific geographic region? Or across the world?
c. Is the 15% drop for a specific demographic group?
d. Did we see a drop in DAU, MAU in the same period?
Did the 4Ps change?
P1 - Did we change the product is anyway that would cause the drop? Is the product still relevant? Did we change any adv. algorithm?
P2 - Did we change any advertising / marketing campaigns that would cause the drop? (Negative emotions)
P3 - Did we change the placements of ads?
P4 - Did internet providers or mobile providers change pricing? Or costs increase for users?
Others potential issues -
Were there any outages with data centers? Or servers?
Did the users start using a competitors / substitute product ?
If the drop is regional
- were there any weather events or acts-of-god during the drop period?
- were there any political events?
- were there religious / cultural / seasonal events?
Once you have identified the root cause you could take specific actions to fix it -
For example, you might realize that the advertising algorithm changed and displaying too many ads within groups. We could fix that.
For example, you might realize that heavy usage of videos/ads could have caused overage charges thereby increasing monthly cost. We could introduce a "low bandwidth mode" for mobile users.