Well, he can't be shy to find a cofounder with an extrovert personality to begin with. He also can't be shy to bargain percentage points; it's all too easy for some extrovert j[oc|er]k to browbeat you into submission; you need him more than he needs you, unless you're Tarmo Uutsalu or Oleg Kiselyov or something.

Perky social web ideas are a dime a dozen.