Okay, speculating about the end of YouTube — the world’s largest, most popular and still most valuable video publisher — is extremely premature. YouTube is where the views are and celebrities are made. It beats MySpace, Yahoo!, AOL and Hulu by many millions of monthly viewers.

But when NewTeeVee reported that NBC is tapped Facebook to premiere its new show, Community, one of this fall’s most anticipated comedies, I had a thought: Did YouTube have a chance at such a plum premiere?

Sure, it had a chance, it’s YouTube. But I’m not sure YouTube is the automatic first choice for premiering video anymore. And I’m not the only one speculating about YouTube’s potential — or lack thereof — in professional and serialized content. For its part, Hulu is adding to its inventory of high quality shows, both new and classic, every week. Hulu is now officially the home for NBC, Fox and ABC’s content. YouTube’s library, meanwhile, still needs work, though it will no doubt improve pretty quickly.

At the same time, as original web series become an increasingly vital part of online video viewing, YouTube is ceding traffic to niche players like MyDamnChannel, Crackle, Revision3 and a host of others that have made names for themselves by curating their programming and hosting a core of quality shows instead of hosting everything. Currently YouTube hosts a few of the breakout hits, most notably Fred, who achieved his celebrity status on the site. But with dozens of popular web shows out there, YouTube knows it could be a bigger player, which is why it recently entered at partnership with Blip.tv in search of good series producers.

Enter Facebook. If YouTube’s value is in its size, Hulu’s value in its professional feel and other sites’ in their management of niches, Facebook’s is in its actively engaged user base. When people know each other and regularly communicate, they are more likely to take each other suggestions. This has been an established phenomenon in communication research for fifty years. Personally, I get more hits from posting a link to my blog on Facebook than posting it to Twitter; my Facebook friends know me and trust my recommendations. If Community becomes a hit on Facebook — not a foregone conclusion, by the way, as users are still unused to watching videos on Facebook — it may translate to TV viewers.

Facebook has an engaged audience, so what, right? Well, as we’re starting to learn from MySpace, size alone (just being where everyone else is) isn’t enough to sustain a site. As I interview editors and producers of online videos, one theme remains clear: an engaged audience is everything. YouTube knows this, adding “Facebook connect” to its video upload page to take advantage of Facebook’s dominance in sharing. Facebook, fully aware of its potential, is fast becoming a player in online video itself — Community is just one of several series, web and broadcast, to use the site as a promotion tool.

While YouTube is too big to lose the top video spot anytime soon, if in five years everyone is watching shows on Facebook, Hulu and everywhere else but YouTube, we’ll know when that started to happen.