We use the Marketo API to get information about our users onto our site, but we're concerned that hitting the rate limit on the site will affect our salesforce sync. Do all integrations share the 10,000 request limit? or is it on a per app basis?
SFDC sync doesn't use the Marketo API.
All your inbound Marketo REST/SOAP integrations do share the 10,000 calls. SOAP in particular has no concept of "app," anyway.
Retrieving data ...