You may see different conversion or lead data in HubSpot compared to Facebook, LinkedIn, or Adwords due to how a conversion is defined between these services.
Note: It is not possible to view existing leads from your ads campaigns in HubSpot. Only new leads created once you have connected your ads account will appear in your ads tool in HubSpot.
You can also read more here about what types of campaigns for each ad service are supported by the HubSpot ads tool.
What is considered a "conversion" or "lead"?
In HubSpot: A lead in the HubSpot ads tool is generated when a new contact submits a form on a page with the HubSpot tracking code installed and can be attributed back to your ad campaign. Existing contacts in HubSpot who fill out your form will not appear as leads for your ad in HubSpot. Read more about how HubSpot calculates leads and customers in the ads tool here.
In Google Adwords: A conversion happens when someone clicks your ad and then takes an action that you’ve defined as valuable to your business, such as an online purchase or a call to your business from a mobile phone. These actions can be customized based on your business, and don't always mean that a user filled out a form. Read more on Google's site here.
Additionally, when reporting cross-device conversions, Google will only include a conversion if Google has at least 95% confidence that the calculation is within 10% of the actual value. Read more here.
In LinkedIn: A conversion in LinkedIn occurs when a user completes the action that you have specified. Read more about LinkedIn Conversion Tracking Reporting and metrics here. LinkedIn also provides some reasons why your conversion may not appear as expected.
In Facebook: Facebook allows users to define what registers as a conversion event. Learn more about these custom events here. For Facebook Lead Ads specifically, those conversions are sent to HubSpot when a user fills out a Lead form. Only new conversions after you have integrated your Facebook Lead Ads account with HubSpot will sync over to HubSpot.