Although the name makes sense with that extension, you're going to struggle to find a fast food restaurant that would brand itself using that name. Don't you think that McDonalds, Burger King, and any other fast food joint out there could probably afford fastfood.com if they thought that was in their best marketing interests?
Hold it and put it up for sale. You might get offers. Could be a future name for a website were you can order food online. Restaurants, fastfood chains can advertise for example. But give it patience as .restaurant is hopefully growing and acknowledged
I believe the potential for this domain lies in the way seo recognizes the tld in the future. It's not brandeable obviously, so it's likely not ever going to fetch millions, but as a redirect to a big name fast food chain who may give a substantial amount for the added traffic it can bring.
Just keep doing what you said you were doing. Developing it to make a list of restaurants and their menus and information about them. You will probably make more money off of ad revenue and other monetization methods than you'll ever make selling it.
There comes a point in time where you realize you have 2 pages worth of appraisals and don't need to bump anymore.
I can't think of many end users as it is a fairly generic term and not something a particular restaurant would advertise on - a lot of restaurants rely on word of mouth and "fast food" is not memorable or distinctive.
You could create a directory type site but you wouldn't invest in a name - you could also choose link, directory, web, or a crappy .com and not lose anything. I think this one is a bit of a loser but at least the term fits so there is the minor hope that someone could approach and take a wrong risk.