There are tons of forums and places online that feature web products and services. It is perhaps one of the most massively flooded markets on the internet. There is no shortage of digital marketing efforts on the web for this sort of product/service.
So your question really shouldn’t be ‘how to promote it’, but rather how to stand out in the crowd. Since you’re specifically talking about youtube videos, that’ll narrow your options a little bit.
If you just want plain old traffic boosts, and you have solid or viral-capable content, you consider outlets like Reddit, Buzzfeed, Digg, etc.
If you have an amazing, cutting edge offering, you could approach big online tech magazines like PCMag, TechCrunch, Gizmodo or Engadget.
For the generic social channels, you already know Facebook and Twitter can be effective, but Google+ and smaller channels shouldn’t be ignored.
Dive into the Blogosphere too, write guest post articles and also promote your own video-promoting articles.
These are all relatively high-level areas to start with. Eventually you’ll need to dig deeper into your niche and seek out places that might not be as big, but still are high quality, high authority and trustworthy places on the web with more relevant content and more focused on your specific offerings.