depends on what you mean by 'vulnerable'... Facebook has the most user information and the least-capable users, so it's probably the best place to fish for personal information.
Twitter has had multiple security issues in the past, mostly revolving around account hijacking and impersonation.
Google/Youtube have a fairly solid history, but the mass of different services attached to a single account leads to a lot of configuration and privacy options that most users won't set up fully, potentially leaking data or leaving the account open to 'theft'...
in terms of hacking the service itself though, good luck. they're all prolific and old enough to be able to avoid the majority of web and network attacks, and any other attacks would rely on very specific knowledge of the service which can't really be compared.
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