Your question is quite broad. I'll copy and paste you an answer of another question which is essentially the same: How can my community grow?
Now we'll head to the disappointing part. Even if you do everything right, you are available, you are open to your community, you can still be not able to succeed. The chances are against you. The biggest factor in community growth is luck. I could list you a lot of start-ups which weren't able to attract customers, they had to reboot several times.
Administering and moderating a community is about your actions, how you interact with your community, how you reward, how you punish. Growth of a community has two factors: your actions and the luck of being discovered. And luck is the heavier factor here.
One positive thing in the end: You'll grow eventually, even if you don't show qualitative videos or streams, there still are people who are attracted to lower quality. You'll grow in the end. How fast is bound to the results of your actions and the odds. And the best thing: As you're getting more and more famous, they factor of luck is steadily decreasing. By getting your name out there, you aren't too depended on luck anymore.
Community growth is something based on luck. There are a lot of methods but you have to realize that your idea is among many others ideas. There are a lot of bytes in the internet, and you are a normal byte as everyone else.
At this point I consider that being the band Mumford and Sons to do concerts in Portugal of around 2500 fans that I should have at least this amount of people to follow the Facebook page.
This assumption isn't right. Although concerts are visited by 2500 people, it doesn't mean that 2500 people use Facebook. Even in this time and age, there are people who don't use social networks because they simply don't like them. Facebook has 1.44 billion monthly active users (March 2015). At an estimated world population of 7.3 billion humans, that just one fifth. So, among five people, there's one using Facebook. This means that just 500 people on a concert actually use Facebook.
Of course, I'll add that you are in a developed country where the use of Facebook is more popular. So, let's say 1700 people who visit the concert use Facebook, it's highly unlikely that they all know of your Facebook page if you're not an official associate of the band. And even if you are an official associate of the band, it doesn't mean that you will gain 1700 likes / followers. There are active, not so active and inactive users. These kind of users have to be added in your calculations, so you'll most likely have around 1400 users. That's also the figure of your current likes.
You also have to consider that there are people who like the music but wouldn't describe themselves as "fans". Fandom is a matter of subjectivity, and there are many ways to express gratitude or obsession through fandom. Not everyone wants to be part of a fan community. How often have you come across someone who claimed that he just is a fan although he's obviously showing obsessive and stalking behaviour? Too often. And this is a negative image of fandom which is widely known. It can harm your growth.
You have to define what kind of fandom you tolerate and accept. By getting away from obsessive and stalking ideas, you'll be able to make your target audience more broad. But you'll also lose these hard-core fans who can contribute to the community in a good way, too. It's just not their way to be in such a casual community. It's a trade-off.
Besides all of that, you have a solid number of likes. 1400 is pretty good considering that the band doesn't seem to be that popular. Even if just 500 people of your fan page visit concerts consecutively, you have reached one fifth of the people attending the concert, meaning that you have achieved a success out of a statistical view. Just keep up your work, and you'll grow alongside the popularity of the band.
(I guess it's worth to mention that you can lose users at any time if the band is pulling of some bad things. Not everyone wants to be connected to a band which has done some bad things.)
I just want to add a quick thought: I think that communities which were created around the term fandom are doomed from the very beginning. They may flourish really well but someday, well, who is this person? Somewhen, there's probably nothing or nobody to be a fan of.