Meh, personally I dislike the idea of building a business on top of someone else's platform. Facebook/Twitter/etc are good ways to reach out to people and spread your message, but it's too easy for them to block you or reimplement your idea.
Found the link: http://techcrunch.com/2010/08/12/twitter-tweet-button/
I do see mentioning of an agreement with Twitter but it is my understanding that Twitter has a licensing agreement to use the tweetmeme technology not that tweetmeme will be powering everything.
Edit: You could be right about the Tweet Button. DataSift will be run on TweetMeme's servers drawing data from the firehose.
At some point the stability, openness, and size are enough to reduce the risk to a very low level. I'm sure it's safe to bet on: Windows, OSX, and Google. I really don't think Facebook is there yet. Not given how quickly we went from Friendster -> MySpace -> Facebook.
Also, Facebook can implement your idea even if you're not on their platform. Look at FB Places. I don't see how using the FB platform makes it any more likely that they will move into your space.
A platform becoming your competitor is fair game, and doesn't imply they will do anything sneaky like making their platform any less accessible to you. Microsoft is in the Operating System business, but MS Office is a fair competitor to other office suites that run on Windows.
I understand the concern about Facebook though, because they aren't (yet) a platform like Windows is. Microsoft started out making the platform, and then built some nice user facing applications to go on top of it. MS Office uses the same OS system calls that are available to every Windows developer.
Facebook started out as a user facing product (facebook.com) and then pushed down into the stack. The issue with that is that facebook.com and other Facebook products are based on leaky abstractions. They don't just sit on top of the Facebook platform. They can do things with the social graph that no 3rd party application can do. This is where Facebook can have an unfair advantage, and why people are worried about building on top of them.
The good thing is that Facebook is working on sealing off the abstraction barrier between their platform and their products. Look at the evolution of their API's. MUCH better social applications are going to pop up once we can all build the same user-facing products that Facebook can build. The great thing is that it's in their best interest. Facebook is much more powerful as Windows than as MS Office.
As far as reimplementing goes, I think they'd just buy you first. They've already invested time and resources to help you develop your app/site, so it will be natural for them just to aquire you.
It's perfect for YC (extra high chance for the startups to be bought by FB) and good for FB (people doing cool, interesting social things they can incorporate). I am still not a fan, but I guess I'm not a huge fan of Facebook anyways.
Facebook is a business. They'll do whatever is in their business interests. If it is "block and copy such and such app" and they can do it without significant reputational damage (which is really the only protection YC startups will have), they will do it.
Sure, but there is a risk/benefit analysis to be done here. If you bet your business on one big company like that, you not only are you betting that the company won't choose to squish you, you are also betting that the company will remain strong enough to support you for as long as it matters.
If you are aiming to build a modest company whose service is fundamentally tied into a currently dominant big player and probably looking for an early exit, that might not matter all that much. On the other hand, if you have a neat general idea and the big company just happens to be the dominant platform in a relevant field today, that doesn't mean they will still be so tomorrow, and your idea might fit with competitor platforms just as neatly.
Personally, of all the companies in the world I would not build my business around today, Facebook is one of the top three. The others are Google and Apple. All three have a demonstrable history of trying stuff and then backpeddling, so if you get taken over, you might get a bit from your options but your idea and future development are hardly secure. All three have a track record of screwing the little guy, sometimes without even noticing, so if you're just building on their platform independently, you're highly vulnerable. And worst of all, I expect all three to take a severe legal and/or market-driven beating within the next couple of years.
For Google or Facebook, it will probably be on privacy grounds. You just can't go around making the arrogant statements their execs keep making, and presuming to collect and sell that kind of personally sensitive data, in a world where the public is increasingly concerned about these things and serious crime built around data loss and ID theft is rising fast. Both companies have already been hit hard by privacy watchdogs in several countries.
For Apple, it will probably be because the walls around their garden aren't strong enough but their corporate arrogance makes them think they are. Steve will probably just wake up one morning and suddenly realise that Android phones or a resurgence from a player like Palm or Blackberry has taken most of his market share, without even noticing how it happened.
Plus of course there are always the basic business threats. Facebook only needs enough people to start leaving that it can no longer function as the de facto standard way for friends to share stuff and it's toast, just like other social networks before it. Google only needs someone else to come up with a better way to do search and it's toast, just like other search engines before it. Apple... Well, they have a prestige brand in the middle of the worst financial crisis in two generations, so you do the math.
To avoid going down with the ship, if I were building a new on-line tool today and needed to work with a big player for platform support, I would definitely want to be working on portable, open standards as much as possible. That is exactly what big players like Facebook do not have, and will try to their dying financial statement not to introduce since it would be anathema to their business model.