Here’s how the math works: Facebook recently acquired mobile photo-sharing site Instagram, announcing that the deal was worth approximately $1 billion. Today it provided more specifics, saying that the deal included $300 million in cash and “approximately 23 million shares” of common stock.

That means the stock is valued at around $700 million, or $30.43 per share. Based on the number of shares outstanding, that works out to a valuation of approximately $75 billion.

But a source familiar with the situation says that actual price per share for Instagram was $30.89 per share (i.e., they knew the exact number of shares, not an approximation), which brings the valuation up to between $76 billion and $77 billion.

This is significantly less than the $100 billion that has been bandied around not only in the press, but on the secondary private markets (where Facebook shares have traded in the low $40s).

Chances are, however, that Facebook is low-balling here. Investor interest in the IPO is expected to be intense, with both a price increase and first-day pop likely. Not only does that mean Facebook could still break the triple-digit mark next month, but also means Instagram might actually have gotten much closer to its original $2 billion asking price than people had previously realized.

[Note: This post has been updated. The original post only was based on the S-1(a) filing, not the additional share information]