I think that, the "Facebook Killer" will be something entirely different to what Facebook is today. The reason is based on a few things but look at all these "Google Killers" - Mahalo, Cuil, Blekko, DuckDuckGo etc with the exception of Bing none are really seen as a threat to Google. Cuil has deadpooled, Mahalo has changed business model and we're only really seeing what Blekko can do which goes to prove the idea, that for something to beat Google/Facebook its going to have to be something new. Such as the evolution, of Facebook becoming a threat to Google so much so they are focusing more on social aspects with +1 etc.

However, going back to your question and looking at it with regards to "open source" issue you raised, its important to note that at Facebook they are very open to the idea of opensource having, released many things into the open source community themselves - http://developers.facebook.com/opensource/ and Facebook even provide a public mirror - http://mirror.facebook.net/ of many open source services. This shows that, an "open source" service won't kill Facebook - look at Dispora (the "open source" Facebook) that hasn't worked which goes to show that, if something is going to "kill Facebook" then it won't look like Facebook.

For instance, Facebook doesn't look like Google but as I highlighted earlier Google are noting Facebook is becoming someone considered a competitor - especially if they move into search - which is why they are focusing on +1 etc.

Additionally, there's actually an interesting article about the Facebook Killer not looking like Facebook posted by Mashable's Pete Cashmore on CNN here: http://articles.cnn.com/2010-11-26/tech/cashmore.facebook.ki... which is also worth a read.