Myself I see two problems here.
First: there is just not enough marketing. People don’t talk about it often enough and in the right places (hint: these are places we want to migrate people from). When someone drops Facebook it is great except if we just disappear from there our influence on those who still stays becomes almost nil. People talk about Diaspora on Diaspora. And sometimes on geeky resources.
Second: most people are scared of all these new terms. They are used to just login and use the network. They don’t really want to learn about mysterious “pods”, “aspects”, “federation” or choose between 10-something projects and 200-something servers. We really need to get this in short and human-readable form. Which probably means dropping all these techy details right where they belong - under the hood.
Just look at Pluspora: in my opinion they were successful exactly because they addressed these two issues. They promoted in their communities. They provided single entry point and told everyone - go here, it will be fun. They attracted almost 12000 users in six months.
Now what can be done about it all. In my opinion the first step would be creating some promotion materials. These materials should offer short, punchy and easily understandable messages. Basically what the main project page says but about five times shorter and with zero technicality. It should pass “grandmother test” - someone who doesn’t even understand computers well shouldn’t be really confused.
Maybe the overall message should be broken into few smaller ones. Right now Diaspora (and other federated networks) don’t offer it. It is either ads with no message at all (e.g. all these Diaspora GIFs and graphics) or wall of text peppered with new concepts. To make things even worse these new concepts are often shown in competing light - e.g. Diaspora vs. Friendica vs. Mastodon and what are intricate but life-changing differences between them.
After we have this we should think about actual promotion. This most likely will require to go into enemy territory - Facebook, Twitter, Instagram. I see tweets in my Diaspora stream all the time - why not do the opposite in controlled and collective fashion? But first we need some soft place for new users to land.
Hopefully the new website for Diaspora project will address some of that but I doubt it will be enough. We as users will have to pitch in.