There is one other relevant difference between metal and cryptocurrency (correct me if I'm wrong): If everyone switched to silver, actual silver would need to be procured and stored. It's a lot easier to procure and store cryptocurrency, to say the least!
I really think you're discounting the network effect that Bitcoin has vs Litecoin (or whatever other currency). The market cap of bitcoins is about $1 billion. Whereas Litecoin's market value is $53 million, making it about 5.3% the value of Bitcoin's market cap. Correspondingly, the amount of businesses that accept bitcoin is substantially higher than Litecoin.
So yes, it's easy to switch from one cryptocurrency to another. But if one currency is worth this much more than another currency, and is accepted by (let's say) also 5% of bitcoin's merchants, why would anyone switch to Litecoin?
You also said,
Lets assume that Facebook is easy to replicate (it's not). If I move to MySpace, I lose all my friends unless they move with me.
If I sell Bitcoins for Litecoins, the risk is minimal and I can exchange it for a similar value.
This is true, but even if there are no fees to move between currencies, which likely isn't going to be the case, you're not taking into account that one currency is accepted by far more people than the other currency. So it's worth a similar value. Great, but now you can only use it at 5% of the locations you could previously. Suddenly I'm not seeing a huge advantage of the altcoins...
(The reason I only mention Litecoin here is that it's by far the most popular of the altcoins. My point is even more true with the other alt coins.)