We cannot afford a lawyer, but could be generous with contingencies.
Move importantly, are you prepared and willing to go to court, knowing that justice (from your perspective) might not be served? It might be healthier to move on. You can still try and compete - not everybody likes Facebook and your app might be better. Just don't let it get to you, and if you can't, walk away.
I suspect you know most of these things on some level, and that this post was more to let of steam. I can only imagine how very frustrating this is. Good luck.
A contingency is meaningless because any firm would look at who they were up against and the cost and the risk would outweigh the payoff.
If you had the money and years to repeatedly sue and appeal, then maybe, some firm would take the case, eventually, as a PR stunt to champion "David" vs "Goliath." That's assuming your claim of infringement has any weight.
Besides, most marketing has some logical path to it. So, if the look and feel matches up with the product in a logical way then it's reasonable they came to a similar conclusion on a parallel path, as opposed to seeing and taking your product, or borrowing liberally from it.
Can you share your product and the Facebook product you think infringes?
If you can't afford a lawyer, create publicity. Give the public the facts so they can make a determination whether they ripped you off or didn't, and whether your product is truly as good as you claim. You can't control which way the outcome will go, but you can do your best to be forthcoming, honest, and play the underdog (which you are).
This way, you could walk away with a ton of publicity and public goodwill, from average users, but also potentially from investors.
I would keep doing what you're doing, maybe start picking out an alternative domain/name in case they would approach you at some point wanting you to change your name.
Good luck with your app.