one potential difference is that it already offers a $5/month paid version, not sure how long it took for Imgur to do that. so at least it has the potential to not need ads ever, but fully dependent on whether the revenue from the paid version can pay for the costs of the free version (500 MB offered for free users).

Edit: but indeed I am usually annoyed by the "Privacy first, no ads forever!!!111" marketing. Unless it's some bitTorrent like technology that decentralizes the load, it's not really possible forever.