I joined 10 years ago after the local equivalent of classmates become utterly useless which mean creators had to seek for revenues and that included but wasn't limited to sponsored profiles of local brands and famous people, virtual currency (you could spend on ridiculous pictures that were meant to enrich your profile and interaction with people), games, annoying promotion of their twitter-alike microblog nobody had idea how to use and which caused great uproar. Service was of course filled with ads of which most controversial one was sucking out your profile picture to dump you an customized credit card ad. Before those "improvements" it was a good place to reconnect with people you knew but since it was something new and unknown, a lot of them had no idea how to use it and made fool of themselves or deliberately used it as dating site - hell, at some point profiles of prostitutes of both sexes started to appear. Facebook largely dethroned naszaklasa in upcoming years but IMO didn't improved social skills of most people who carried those along to the new network. The service was sold but still operates and it's popular among less experienced with technology people.

The similar scenario happened with news agregator similar to digg - at the beginning wykop was aimed for powerusers, IT professionals but quickly idea was extended and included content of various type. Userbase grow had an upward trend which of course lead again to monetization; ads, sponsored content, microblog, shameless promotion of certain political agendas were introduced and at the same time, the content quality heavily piked down. Site still operates today under third - if I'm not mistaken, owner but I'm no longer there since interaction with biased content and teenager, 20-something trolls is not appealing at all.

So yeah, I believe it's pretty the same thing everywhere: a simple service idea is successful, userbase grows and revenue sources are needed. Sources are being introduced along with new features but content quality starts to drop. Unpopular decisions are made leading users to migrate in search for better and simple alternatives.

Do you think there is enough abandonment of the core userbase that Facebook can be dethroned?

I really doubt it; they may lost users in long term but not because of new player - they have seem to reached already over-complexity stage with features and thus service will simply become unappealing to existing and new users but that will take time. If they start changing messenger into more discord-slax alike service then they may keep users as this form of communication seems to be getting more popular