According to several execs who contacted me today, this is what Yahoo (YHOO) did in late 2006, when the troubled Internet portal was starting to suffer from drift and its stock was struggling. The solution was “Project Engage,” which was a combination of granting options and restricted stocks units (RSUs).

At the time, employees were given two of each kind, with the stock options and one RSU grant having a longer vesting timeframe (typically several years). The other RSU grant, which is essentially outright grant of stock, actually just vested on Feb. 2, which might explain some recent departures of top talent to new jobs.

Said one exec: “Everyone was just biding their time for the RSU to vest and the Microsoft bid just gives everyone an excuse to leave, because it is hard to imagine wanting to work for Yahoo if it gets forcibly taken over.”

The new Yahoo retention packages would again presumably help hold onto talent, if the deal does not go through, while the enhanced severance would give them a comfy escape route if Microsoft (MSFT) takes over.

As an added benefit to Yahoo, which is seeking to escape Microsoft’s embrace, it will make acquiring Yahoo even more pricey for Microsoft. In addition, if it wins Yahoo, the software giant will still have to hand out even more retention benefits to stave off an exodus.

In a follow-up to BoomTown’s story, Kevin Delaney of The Wall Street Journal has more details of the severance plan, which will cover all employees even if they are laid off due to a change of control at the company.

Yahoo CEO Jerry Yang alerted employees to the severance benefits plan in an email Friday, promising specific details would be available to staff today, the story said.

That’s because, although it has been widely reported, the hot social network will not fall under the Securities and Exchange Commission’s old “500 shareholder rule,” which would have forced it to report such information when the number of shareholders, including those holding just stock options, reached 500.

While companies like Google got caught in that net several years ago, the SEC actually recently changed that rule to exempt those just holding options, which the vast majority of Facebook’s growing legion of employees (about 400) have been given.

In fact, said many sources, Facebook only has several dozen individual shareholders who actually own stock and that number is unlikely to rise in the future.

That’s because new employees, for example, since the start-up’s recent $300-million financing that put its valuation at $15 billion, will be getting mostly “RSUs,” or restricted stock units–grants of stock valued in terms of company stock, which are not issued until later.

These units are essentially a promise that employees will receive stock in the future, but frees them from vexing tax problems such as alternative minimum tax issues.

That also means few will be able to sell any Facebook stock until there is an actual liquidation event, such as an IPO, which company investors like Peter Thiel of the Founders Fund have said would not occur until at least 2009.

Until then, don’t expect much selling from major owners, such as CEO and Founder Mark Zuckerberg, who owns 20% of the company outright and has options representing another 7%.

While it’s a good story to imagine them trying to hedge their bets on the future of Facebook, many sources have told me neither Zuckerberg nor other top execs have done so.

So Valleywag’s Owen Thomas (pictured here in this lovely self-portrait) was right to retract his story over the weekend about Zuckerberg selling $40 million worth of his own shares in the massive financing the company recently got from Microsoft and Asian billionaire Li Ka-sing.

I was in the unusual position of getting the story sent to me by Thomas on my Blackberry Friday night, while at a holiday party that Zuckerberg was also attending.

So what did I do? Well, shove the device in Zuckerberg’s face, of course!

Zuckerberg took a look at the story and quickly denied cashing out in any way, saying to me: “Well, that’s just not true at all.”

In yet another bizarre Yuletide twist, I was headed to the Valleywag party right after that, so I told Owen–who was looking dapper in tux and red shades–that reaction (well, I kind of yelled it at him, to be honest, starting with a festive “Oops, you’re wrong!”).

In other words, it was too good to be true. And, as readers of this column know, so will Facebook’s valuation be until it garners those hoped-for advertising revenues to back up the big numbers.