Since it started it's obviously stronger. Since it's prime? Well, it's decayed a bit, but that's just part of the game. There will always be people who dig something because it's here and now. As time passes, the fad-followers and people who came in strong and burned themselves out will naturally fall away, leaving behind the people (ponies?) left in it for the long game plus some natural churn as new people are introduced (or grow into the fandom as unholy nerd-spawn) and people leave it for various reasons.
These days G4 pony is in the long haul phase. After the initial surge left we didn't collapse into nothing, which is a good indicator for our long term survival, and the most likely path is the same route that shows like Dr. Who and Firefly took where we'll never be a Star Trek/Star Wars level fandom, but we'll never really die.
The fandom has had its ups and downs, but I think it's about the same. I think the hype over it might've calmed down for the most part and I think now people are more accepting of us in society than when we introduced to society. Personally, I'm in this fandom for the long haul and will stick with it to the end. I don't know what I'd do without it now, honestly. I love the art, music, videos, etc that we all produce and it never ceases to amaze me.
I think that the fandom seems a bit quieter since it's no longer expanding explosively. The number of people who count 'brony' as one of their defining characteristic is perhaps lower than it used to be, but the number of people who might admit to being a pony fan is likely as high as it's ever been and still growing.