It's a sign that project could turn into a scam, but just to make sure then you should check for every social media available on that project, maybe they are not that active in Twitter but still answering some questions on telegram for example. Although, still it gives you some red alerts from that inactivity of that one social media.
Otherwise, If the same issues still persist without any answer, then just go away from that projects because it's highly chance to be a scam.
That's mostly where it is going. Those projects that are no longer active on their social media accounts are likely have left it and abandoned it.
They're done and the project's done so don't burn your money buying the project they've got or tokens that it has got.
You're just close to losing your money immediately if you're not careful with those. Just leave them quickly and don't give your time to them if you haven't invested yet to it.