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. REUTERS Ackman stated that he wanted to model the fund on Warren Buffett’s Berkshire Hathaway, a permanent-capital machine that can ride out storms and compound for decades. “This is the first time someone with $50 could be a long-term shareholder,” Ackman told CNBC’s “Squawk on the Street” on Wednesday.…