Zynga is planning to let at least some employees sell a portion of their stock in the hot social gaming company to outside investors, according to two industry sources. Institutional Venture Partners, a late-stage venture firm that has already invested in the company, is the purchaser of all the employee stock going up for sale, one source tells us.

[Updates: Zynga has announced $180 million in funding led by DST, with hedge fund Tiger Global, Andreessen Horowitz, and IVP participating. The New York Times has also just published an article interviewing Zynga executives and board about the deal. The employee stock sale is on, and Zynga employees will be able to sell 10% of vested shares, at $19 per share, according to a new source. One of our previous sources clarifies that IVP has separately been buying up Zynga stock on secondary markets, while another source adds that $50 million has been allocated for the Zynga employee stock sale.]

We have asked Zynga for comment on these rumors multiple times in recent weeks, but we’ve gotten nothing back so far. However, given what we’ve heard, and what we know about Zynga’s strategy, an employee stock sale makes sense.

Facebook, home to tens of millions of Zynga users, let employees sell some stock this past summer to Russian investor Digital Sky Technologies. That move, and this one, are intended to let employees make some money from their hard work while inspiring them to stay around for a long time to come (and make money from the rest of their stock when the company goes public or gets sold). Neither Facebook nor Zynga are looking to make an initial public offering any time soon, we’ve heard consistently from sources in the last few months.

The gaming company is also hiring talented engineers and other employees quickly. It now has more than 600 employees as well as more than 300 independent contractors, the company has said. We and others estimate that it is on track to make more than $200 million this year, with some estimates we’ve heard going higher than that. Most revenue comes from selling virtual goods in a wide range of hit social game, like Texas Hold’Em, Mafia Wars, FarmVille, Café World, FishVille and most recently, PetVille. It also has large games on MySpace, other social networks and some on the iPhone.

On Facebook alone, we track 37 of Zynga’s dozens of applications through AppData. They combine for an unduplicated total of 232 million monthly active users, although this total does not reveal the number of users who play more than one Zynga game. A more interesting measure, perhaps is the fact that it has a combined 64.6 million daily active users on Facebook, or around 19 percent of Facebook’s monthly active users.

Although the company, like others in the industry, has run some scammy advertising offers in the past, it decided to pull them all last month. At that point, it said it planned to keep offers out “until we can control their inclusion and presentation ourselves,” according to chief executive Mark Pincus. This means the vast majority of the company’s revenue is coming from direct payments for virtual goods.

However much money Zynga is bringing in, it is also facing competition from many other growing social gaming companies — companies that are also making money, and can afford to pay well. This employee stock offering is not just a way to keep employees happy, but to preclude them from getting recruited by competitors.

Long-Term Plans

We don’t know how much of their stock Zynga employees are selling, nor the price (assuming that what we hear is accurate). However, we do have a couple other details from one source. Zynga itself may have raised a new $25 million round of funding at a $1 billion valuation — we’ve also heard that this is “way off.” Rumors about a $1 billion valuation have gone around already; we heard it in October, when rumors of a Zynga IPO were also circulating. Others have heard or estimated a similar valuation.

Separately, a regulatory filing for $15.2 million in funding surfaced in November. This was an extension of the company’s second round of funding; and, the money was actually raised in June at a $625 million valuation, according to one source. The $25 million, according to the source, is a new round.