A LinkedIn for spies would be a professsional social networking site where everybody lied about who they were and what they had worked on.

So LinkedIn, basically.
“But I don’t lie on LinkedIn” is exactly what someone who lies on LinkedIn would say.

(It’s also what someone who didn’t get the joke would say.)
@maxleibman I always say that LinkedIn is the most honest social network because everyone is lying but also everyone knows that everyone is lying so it evens out.