There’s waterfall.social which has been made open source (Idr under which license. I think it might be AGPL?) Not sure whether that’s what you mean by “open pla

There’s waterfall.social which has been made open source (Idr under which license. I think it might be AGPL?) Not sure whether that’s what you mean by “open platform”. It also has a number of nice features added compared to tumblr, like changing which blog of yours is the “main blog”, having a separate dashboard and following list for each blog in an account, a system to detect uncredited art-re-uploading, etc.