Rather than adding dedicated Tumblr, Instagram, etc fields, how about just having more "website" slots? I'm sure that should be fairly easy to implement, at least compared to my other suggestion (textbox of network-username pairs). For IM programs, protocol-specific fields allow the forum to build specialised program-launching links, but for social networks like Facebook and Instagram where it'll just be linking to a webpage, there's no benefit in having a dedicated field, those can all be treated as regular websites. The one website slot that's there already lets you set a URL and a Title. For social networks, people could put the network name as the title (e.g. "Facebook").
The lone website slot made sense back when many people had a personal website and before social networks, but nowadays most people have a number of "websites" in the form of social network pages like dA, FB, etc.