Of course it much depends on content and revenues, server hosting, etc., but still how many posts in xenforo based forum (approximately) do you think shared hosting can handle until moving to either VPS or dedi is needed.
Ill ask more detailed question. We are just to meet 600k posts and we have dedicated server. However, advertisement network is lagging our portal so much, that it is irritating in addition that dedicated s. costs hardly justify with revenue.
So, 600-700k posts forum if xf is platform, can this be handled by shared hosting?
Whats the spec of your current server and what are its load averages like? What sticks out to me here is you have avery high ammount of bandwidth being used, I have more users than that online and more content, and use a fraction of what you do.
Shared hosting is definitely a no-go for you. As slavik said, you'd do well with a solid reputable VPS. VPS are easy to scale too so that's an advantage if the price of a dedicated server puts you off right now. I'm on a dedicated server for $40 USD a month, 1GB ram, Core 2 duo processor, 120gb HD and I've done really well with my host but the catch to mine is that it's completely unmanaged, so unless you're comfortable with configuring the server and troubleshooting everything on your own (or have helping hands to do it), I wouldn't recommend what I'm doing.
You might consider a cPanel-based server, VPS or dedicated. cPanel manages most of the functionality you need for your server, and you hope the host can do the rest. cPanel also has great tech support, so the combo should be enough even if your command line capabilities are challenged.