Facebook is not going to be supplanted by Twitter or iCloud, neither of these serve remotely the same purpose.
...and neither will HTML pages supplant desktop applications or brick-and-mortar stores, right? Because neither of these serve remotely the same purpose?
That's exactly the way things often go in technology and business. New wins over incumbents not by attacking them head-on on their home turf, but by creating a new niche with some interesting, unforeseen properties.
At some point commerce realizes world + dog uses it and start providing goods and services -- and the new is thence understood to have supplanted the incumbent.
Facebook is pretty much the landline phone of late XX century. The market's ripe for something like the cellphone to disrupt it.
The only instance where I thought that Facebook was useful was for getting together for a 10 year high-school reunion, but even then, only half of my old colleagues were on Facebook. What worked was plain word-of-mouth, since somebody always knows someone that is missing from the conversation and can get in contact.
Basically what has the potential to kill Facebook are the old means of communication, channels that are still reliable and still widespread. That's why Facebook does provide IM and even email, as they want to move your Inbox on their servers; but IMHO, that remains to be seen how well it plays out, as people are pretty conservative about their current email and IM accounts (it's like your phone number, you don't want to change it so often).
And I can see why they are afraid of Google - Google has users that rely heavily on their email service, which is state of the art IMHO, it has user profiles that will appear in search results, it has productivity apps that allow for collaboration, it works on social search features as an alternative to Facebook's Like, and if Buzz wouldn't suck so much, they would have been a serious threat by now.
Unfortunately for Google, they can't seem to pull their shit together. Also, judging in this light, Microsoft's acquisition of Skype may not have been so stupid after all.