One plausible risk scenario is, under pressure, they try make significant changes without incremental testing.
I don't see a good alternative channel to Twitter right now. Facebook certainly isn't it. Instagram definitely sliced out some of Twitter's market, but can't do the same in other use cases. Neither can Snapchat.
From the standpoint outgoing traffic Facebook dominates Twitter. That is a concern. Fake traffic or real, this is a signal of engagement.
Another problem is ad platform dominance. Advertisers will spend on well built platforms with large audiences. Google and Facebook clearly excel in this area now. It isn't so much Google & Facebook that threaten Twitter but all the other social apps and sites which are banking on the exact same model to justify their valuations. If Twitter is earning 1/3rd or 1/4th of the amount per user Facebook is in the long term this could be a problem.
There is a price where Twitter becomes a very strong acquisition target. It certainly is heading in that direction.