Bill Ackman ‘s long-anticipated Wall Street debut flopped on Wednesday, as shares of his new closed-end fund tumbled 18% on the New York Stock Exchange despite the hedge-fund billionaire’s rampant following on social media.

Pershing Square USA Ltd. (PSUS) — the flagship vehicle pitched as a way to hand hedge-fund-style returns to mom-and-pop investors — opened at $42 a share, down 16% from its IPO price of $50 — and drifted lower to close at $40.90.

The IPO and an accompanying private placement had raised $5 billion Tuesday night — the largest-ever for a US closed-end fund.

Ackman’s new investment vehicle launches after a two-year delay owing to weak initial demand among investors.…