They would likely work but very slowly. Depending on settings, Facebook can use tons of data due to auto start video and other things.
You mentioned being told of 5-8 Mbps speeds ... there are some Hughes plans that top out at 5 Mbps.
There is a misunderstanding going on here somewhere,
Here in part are some pertinent portions of the Subscriber Agreement:
How does the Fair Access Policy work?
HughesNet Gen4 service plans allow unlimited downloading and uploading. However, to ensure consistent performance for all users, a Fair Access Policy provides for a limit, based on your service plan as described above, on the amount of data which can be used before your service performance is reduced. The policy is implemented automatically by monitoring your usage, including data which is sent from or to your HughesNet terminal. Each HughesNet Gen4 service plan is assigned an Anytime Allowance. The Anytime Allowance is the amount of data, in bytes, which can be uploaded or downloaded within the month at full speed. If your data usage exceeds the allowance for your plan, the throughput (speed) of your service will be reduced
If I exceed my Data Allowance, when will my speeds be reduced?
During the Bonus Period, your service will operate without restrictions if you have Data Allowance available in either your Bonus Bytes or Anytime Allowance. Outside the Bonus Period, your service will operate without restrictions if you have Data Allowance available in your Anytime Allowance. If you do not have bytes available in the period, your speed will be reduced to approximately 150kbps. As an example, consider the following scenarios:
1. Your plan provides a 10 GB Anytime Allowance, a 10 GB Bonus Data Allowance, and a Bonus Period of 2 a.m. to 8 a.m., local time. You have used all 10 GB of your Anytime Allowance by sending and receiving data outside of 2 a.m. to 8 a.m., so you have 0 bytes remaining. Outside the Bonus Period, from 8 a.m. to 1:59 a.m. local time, your service performance is reduced. During the Bonus Period, your service performance is not restricted and your usage will be deducted from your Bonus Bytes.
2. Your plan provides a 10 GB Anytime Allowance, a 10GB Bonus Data Allowance, and a Bonus Period of 2 a.m. to 8 a.m., local time. You have used all 10 GB of your Bonus Data Allowance by sending and receiving data overnight from the hours of 2 a.m. to 8 a.m., so you have 0 Bonus Bytes remaining, but you still have 5GB of Anytime Allowance remaining. Outside the Bonus Period, from 8 a.m. to 1:59 a.m., local time, your service performance is not reduced, and your usage is deducted from your Anytime Allowance. During the Bonus Period, your service performance is not restricted, because your usage will be deducted from your Anytime Allowance.
Who is affected by the Fair Access Policy?
Some users consume much more bandwidth than the average user, and they will experience reduced download speeds as a result of exceeding their Download Allowance. You may be surprised to find that the top one percent (1%) of users download 9 times more the average user. By providing a Data Allowance, more of the shared bandwidth is made available for everyone to use. Most users will have a better experience as a result of the Fair Access Policy.
Online activities such as viewing Websites, checking email, watching video clips or similar short streaming media, and automatic software updates are unlikely to cause you to exceed your monthly Data Allowance.
Typically, on the Prime or Prime Plus plan, to reach your Data Allowance in a one-month period, you would have to download any of the following:
What will my Internet access be like if I exceed my Data Allowance?
When your Data Allowance has been exceeded, you will experience reduced download speeds throughout the balance of the month. During this period, your HughesNet service can still be used for activities such as Web browsing and viewing emails, but speeds will be significantly slower than your standard plan rate.
If I exceed my Data Allowance, what should I do?
You might consider upgrading your service plan.
How do I verify whether I have exceeded my Data Allowance?
You can track your past usage and Fair Access Policy status by going to www.myHughesNet.com, clicking on Customer Care, then clicking on the Check My Usage link in the Resources section.
What do you recommend I do during the Bonus Bytes period (2 a.m. to 8 a.m., local time for consumer; 8 a.m. to 6 p.m., local time for business)?
We recommend that you automatically schedule your system updates during this period. If you install HughesNet Tools, your Windows Updates will automatically be scheduled for this period. You can also use a download manager software application to automatically schedule large file downloads to occur during these hours.
The Subscriber Agreement can be read in full here:
Just as your car will slow and then stop if all the fuel is used your Hughes service will slow dramatically when all data has been used.
There are three usage meters to use in tracking your data consumption.