I don't see how the need for a phone follows from being a major internet player.
Apple built a phone to build a better phone.
Google built a phone to make sure they had a solid foothold in mobile ads and the mobile web in general.
Microsoft built a phone for mobile ads and to ensure that their enterprise applications had a mobile experience that they controlled, so no one would switch to a different application because it had better mobile features or support. It should be noted that Microsoft's efforts are failing, though their tablet will probably see some success. (EDIT: That's clearly not the reason why Microsoft built phones in the first place, but I think it's a solid justification for why they still care. Windows Phone is a money loser thus far.)
Facebook doesn't have a phone. They're rumored to be building one. I don't understand why it's a good investment for them when their apps are on every phone, with contacts integration on iOS.
Amazon is rumored to be building a phone, but I don't understand the logic behind it. The only reason they have a mobile OS is because they needed a device to push their content to.
Other than advertising, I don't see a good reason for Yahoo to start building devices that are any more involved than throwing some Yahoo apps on top of Android. If they built something incompatible with an existing mobile OS, it would fail.
The reason for Amazon to build a phone is for distribution of digital content. As Apple proved with the iPod and then the iPhone, the device is the distributor. Music, movies, video, books, and apps are all multi-billion dollar industries. In a fragmented world of content producers, power goes to the distributor. The decreasing costs of producing quality content means the world will only become more fragmented, hence distribution becomes more critical.
I guess I was thinking mostly about books. A phone makes sense since Amazon sells a lot of content that people consume on their phones, particularly music, and to a lesser extent, movies and books. As a defensive move against Google Play, it makes even more sense.
My comment aside, you are right to question if it's the wisest investment for Amazon to make a phone. Could they form strategic partnerships instead, paying handset manufacturers to install Amazon media marketplaces like Google did with Dell to install search? As Nokia, Palm, BlackBerry, and others can attest, phone making is a difficult business. I haven't crunched the numbers and cannot offer an informed opinion, but as I articulated above, there is reason for Amazon to consider the option.
Apple built a phone to build a better phone.
Google built a phone to make sure they had a solid foothold in mobile ads and the mobile web in general.
Microsoft built a phone for mobile ads and to ensure that their enterprise applications had a mobile experience that they controlled, so no one would switch to a different application because it had better mobile features or support. It should be noted that Microsoft's efforts are failing, though their tablet will probably see some success. (EDIT: That's clearly not the reason why Microsoft built phones in the first place, but I think it's a solid justification for why they still care. Windows Phone is a money loser thus far.)
Facebook doesn't have a phone. They're rumored to be building one. I don't understand why it's a good investment for them when their apps are on every phone, with contacts integration on iOS.
Amazon is rumored to be building a phone, but I don't understand the logic behind it. The only reason they have a mobile OS is because they needed a device to push their content to.
Other than advertising, I don't see a good reason for Yahoo to start building devices that are any more involved than throwing some Yahoo apps on top of Android. If they built something incompatible with an existing mobile OS, it would fail.