Silencer's lack of mobility and frailness makes him a questionable carry, and his recommended itemisation lacks attack speed, whilst the complete absence of lockdown or stuns also makes him fairly ill-suited to a traditional support role.
He is usually therefore relegated to 4th position semi-support/semi-carry, initially taking on joint support duties while later transitioning into a semi-carry role as he accrues intelligence.
Skills
A solid all-round build is:
Skill and max Last Word first. Max Curse of the Silent second, and lastly Glaives of Wisdom. One early point in Glaives is great for lane harass. Always skill Global Silence.
If lane opponent(s) do not have spammable, low mana cost abilities:
Skill and max Curse of the Silent first, followed by Last Word and finally Glaieves of Wisdom. Always skill Global Silence.
Items
Starting Items: Observer Wards or Courier, Tangos, Healing Salve, 3x Iron Branches, Clarity.
Early Game: Observer Wards, Boots of Speed, Magic Wand.
Core Options: Arcane Boots (utility) or Power Treads (semi-carry). Mekanism (utility) or Force Staff (semi-carry).
Luxury Items: Drums of Endurance, Orchid Malevolence, Scythe of Vyse, Black King Bar, Rod of Atos, Shiva's Guard, Aghanim's Scepter, Ghost Scepter.
Situational Items: Boots of Travel (late game), Necronomicon (pushing line-up).