Why are my audience sizes in Facebook or Google Marketing Platform smaller than expected?

Both of these partners will only add audience members downstream if the data you're sending matches what already exists in their system. 

Google Marketing Platform

GMP only accepts IDFAs and GAIDs from mParticle.

It's also worth checking "Membership Lifespan" in your mParticle connection settings - if this is shorter than your audience retention period (ex. 30 days), then users may have dropped off in Google.

Facebook

Facebook accepts multiple identity types and attributes for matching in their system. If you only send one or two identity types, then it's likely your match rate will be low. Try adding more identity types or attributes for improved match rates. 

For matching, Facebook accepts the following identities:

1. Email
2. Facebook ID
3. IDFA
4. GAID
5. Phone Numbers

and the following attributes:

1. First Name
2. Last Name
3. First Initial
4. City
5. State
6. Zipcode
7. Country
8. Gender