Solana (CRYPTO: SOL) has been one of the surprising crypto success stories of 2023. After a disastrous 2022 that saw it lose 94% of its value, Solana is churning out triple-digit returns for investors this year. Solana now trades for $22.35, more than double its value at the end of 2022.

While Solana's performance has been impressive, it's time to look under the hood to see whether this performance is sustainable over the long run. If you are thinking about buying Solana right now, there are two key figures to consider.

Solana vs. Bitcoin

For the year, Solana is up 127%. That performance is better than any other cryptocurrency currently ranked in the top 25 by market capitalization. In comparison, Ethereum (CRYPTO: ETH) is up "only" 59% and Bitcoin "only" 76%. On the TradingView chart below, it's possible to see just how significantly Solana has outperformed Bitcoin this year.

That's why I'm concerned that Solana's performance in 2023 might be unsustainable. Does anyone really think Solana is a significantly better investment than Bitcoin right now? My hypothesis is that Solana's price is simply returning to its levels before the FTX crisis hit.

Back in November 2022, investors were tremendously concerned that Solana would be the victim of FTX contagion due to its close relationship with former FTX CEO Sam Bankman-Fried. As a result, Solana's price fell to under $10 for the first time in nearly two years at the very end of 2022. Thus, what looks like tremendous market outperformance this year could really just be Solana retracing its path back to where it was six months earlier.

NFT sales volume

To examine this hypothesis in greater detail, I checked out the latest non-fungible token (NFT) sales volume data from CryptoSlam. After all, NFTs are the lifeblood of the Solana blockchain ecosystem, and a key selling point of Solana has always been its NFT growth story.

The encouraging news is that Solana still ranks as the No. 2 blockchain when it comes to NFT sales volume, trailing only Ethereum. But it's not even close. Over the past 30 days, total NFT sales on Ethereum were more than 5X those of Solana. So Solana is not getting any closer to unseating Ethereum as the NFT leader.

At the same time, a number of Ethereum Layer 2 blockchains -- such as Polygon -- are growing very rapidly. Over the past 30 days, Polygon's NFT sales volume is up 35%, while Solana's sales volume is essentially flat (down 0.37%).

Moreover, if you dig a little bit deeper, it's getting increasingly harder to find any Solana NFT collections that rank in the top 20 by sales volume. While Ethereum has name-brand collections, like the Bored Ape Yacht Club and CryptoPunks, the top Solana NFT collection right now is Mad Lads, which only launched in April. Two of the best-selling NFT collections for Solana abandoned ship last year in the wake of the FTX meltdown, and Solana has been struggling to find a new flagship NFT collection ever since. The hope now is that Mad Lads (currently ranked No. 6) can play that role.

Buy, hold, or sell?

Based on the above numbers, the case for buying Solana is more complex and nuanced than it might appear at first. From my perspective, Solana's price has over-corrected to the upside this year after over-correcting to the downside last year. Solana's price seems very volatile right now, and I'm especially concerned that Solana has overheated to such an extent that it's significantly outperforming Bitcoin.

If Solana's NFT sales numbers were a bit more encouraging, I'd be OK with that. But for now, Solana is a hold -- not a buy. It's worth holding on to Solana to see whether it can reclaim some of its mojo from the previous crypto bull market rally. But I cannot recommend it as a buy until Solana's NFT numbers show better performance.