I think it's more the "question & answer" sites like StackOverflow that have taken traffic away from forums, probably not for all subjects though
That and reddit.
(frustrates me no end to find a technical/how-to post where linked images formed a substantial part of the information and all I can see now is "Image Not Found" placeholders)
Likewise, BUT on the other hand, I have to keep hosting costs at a minimum, so... I decided to compromise... until the former became too frustrating for me as well.
Other than reactive themes, has the SMF development team considered making the platform more mobile-friendly? I think if SMF was more geared towards mobile, while also respecting PC users, it could boost activity.
Depends on the type of forum you're running, i.e. what the forum's about, but in my experience, no, it won't. I've seen really good looking forums, mobile device friendly and all that, drown. It doesn't make a difference, the user base is the problem. Everyone's expecting forums to look more like social media, because that's like "the norm" - if it doesn't have a like button, it's not worth visiting

.
The reality is, people have gotten lazier and lazier... everyone wants everything to look fancy and have all of their info and "toys" in one place. Social media achieved that about the "toys" part, but since most people are not critical thinkers, the info part is disregarded and "well... let's just see what's everyone doing and click away" is the thought that prevails.
Ultimately, I came here for advice, as someone who is very ambitious and wishes to start their own community, but has a nagging feeling that it may just not be worth it.
Unless you're planning on spending 5 to 8 hours a day creating original content and tweaking your forum to cater the needs of the users, rather than having a life of your own... no, it's not worth it.
See, the problem is, social media and other platforms have raised the bar when it comes to "how interesting your site/forum" has to be in order for it to be "busy"... and obviously, you as an individual (one man show) can't possibly compete with a company with thousands of employees. These are the cold hard facts. Most forums are one man shows (in some rare cases, 2 to 3 people might run it), so, it's inevitable to always be lagging behind a company with thousands of employees.
The internet nowadays is more like a shopping mall. If you don't rent a space inside the mall, you're not worth visiting, plain and simple.
Number one reason is that not everyone wants, or is allowed by their employer, to have a Facebook account...
This might be true in the US and a bunch of other western countries, but trust me, this is not the case in most eastern countries.