Development Priorities

I'm an avid user of a couple social media sites, like Facebook, Twitter, and Tumblr. Every so often, these websites make a functionality update, and usually I am left scratching my head wondering what on earth made the devs of the website make those decisions. I'm not the most experience when it comes to web development, but I like to think that I usually give devs the benefit of the doubt when they implement new things, but more often than their ideas seem to come out of left field.
For example, it was discovered today (ie, Twitter did not officially announce it but their strange methods of operation are another story) that Twitter is adding a feature for users to receive direct messages from anyone. Considering the nature of Internet users, ie more than willing to dump their opinions on anyone with 9000 times the vitriol than is necessary, I am very curious about what inspired the Twitter devs to implement this feature.
Tumblr is particularly infamous (at least with regards to its user base) for implementing updates that seem useless, while ignoring features that seem to be universally desired. Most of their updates, for example, seem to make pointless rearrangements of their UI, whereas they lack a 'block' feature, which many users want. It is possible that they could have something like this in development, but one would hope that, since so many people are asking for such a feature, the devs could at least say that they're working on something that their users actually want.
Obviously these companies - particularly Tumblr, since its owned by Yahoo - have ulterior goals that they're trying to reach by catering their website. Tumblr, for example, is trying to become similar to Medium, ie a long-form blog website. I find it odd that they're trying to change their functionality so late in the game; Tumblr already has a strong user base that is happy with the website's core functionality, why would they try to change it? They could, of course, be trying to attract new long form blogging users, but then, why wouldn't said users go to Medium, since that website already does what they want? I suppose, though, that "we already have users, let's just sit pretty" is a rather strange business mentality, and stagnation can lead to death in these situations. While these websites try to cater their services for what seem like pointless reasons, I'll just sit here and complain while I continue to use their sites.