in my current company we are using Salesforce and HubSpot, and we have synchronized Contacts and Leads.
HubSpot does around 7000-8000 daily api calls, but I do not know what is the frequency HubSpot connects to Salesforce, or if there are some ways to reduce this number.
Thanks.
Salesforce limits its API calls with the edition and licenses you have see this link
(https://help.salesforce.com/HTViewHelpDoc?id=integrate_api_rate_limiting.htm) and for reducing the api calls from Hubspot end follow this link (http://knowledge.hubspot.com/salesforce-integration-user-guide/how-to-adjust-your- salesforce-api-call-limit)