Reddit and Twitter embraced third party clients because it saved them the early cost of doing it themselves and provided extra avenues and incentives for power users to create content and moderators to cultivate communities. Both benefit the platform and in the end revenues.
Social media platforms are almost always generous in allowing for tools and third party clients that result in new content being created. What they restrict is the same sort of thing for the passive viewing side.
Reddit and Twitter were always unusual in that there’s such a 50/50 split between posting and consuming, and power users valued being able to curate their reading experience as well as the tools they had to post with. Most of Twitter’s core feature set came from power user features invented in third party apps, including retweeting. But as the apps became more popular they also took away a larger proportion of passive eyeballs for advertisers. A balance of some sort was needed, and I think both Twitter and Reddit have gone the wrong way.
Facebook is different to those two in that it always had the benefit of exploiting your existing in person social graph and growing outward from there, meaning it had different drivers and incentives for frequent posting.
