Zynga will sink or swim on its own, but its IPO could be important for Facebook-dependent companies that follow.

Social gaming juggernaut Zynga plans to file for an IPO this week, according to CNBC’s Kate Kelly (who seems to have found a steady leak within lead underwriter Morgan Stanley). It would look to raise between $1.5 billion and $2 billion, at a valuation of between $15 billion and $20 billion. This would represent up to a 25% premium to where Zynga has been trading on the secondary markets, and could serve as a portfolio alternative to traders scared of getting into Groupon.

Whenever we do get to see Zynga’s S-1, expect most of the criticism to focus on its Facebook dependency (there always is criticism of a new issuer, no matter how successful).

My best guess, and it’s only a guess, is that this line of attack will be short, swift and forgotten. Fusion-IO (FIO) was largely reliant on Facebook from a client perspective, and that hasn’t hurt the stock. Demand Media DMD admitted potential disaster is Google GOOG tinkered too much with its search algorithms, yet still priced well (although it had struggled in the aftermarket).

The real question is if an Internet company like Facebook can be widely accepted as a legitimate, stable platform upon which other publicly-traded companies can build. Kind of like how the Microsoft or Apple operating systems are viewed today for software makers.

Zynga will be the largest test to date, and it clearly has passed the private-market preliminaries. Its IPO will be the medal round.